Oil Change Interval for Synthetics

Hi all, I'm thinking of changing my '90 Acura legend over to a synthetic oil product (Mobil 1 Amsoil..etc..suggestions?) and was
wondering what intervals I can expect between changes. One person I was talking to said they can go to 25k. I don't know if I'd be comfortable with that, though. The car currently has only 88K on it now, but I put about 4k on it a month. Thanks for any input!
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Why would you change the interval at all?
Synthetic oils won't break down as easily, it's true. But the viscosity modifiers are the same as with conventional oils, and they will break down just as quickly. Synthetic oils can't hold any more combustion byproducts in suspension either, so if the limiting factor is the amount of junk in the oil (as it is for most older engines), that's unchanged. In fact, the better solvent properties of synthetic oils may cause them to get more dissolved junk in them, when used on older engines. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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On Aug 20, 2:35 pm, snipped-for-privacy@panix.com (Scott Dorsey) wrote:

I just don't understand why, then, do they tout it go for so many more miles between changes than conventional oils. I do agree about the particulates in suspension, though....
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The Doctor wrote:

I somewhat disagree with Scott. The additive packages are rather different on synthetics- for example most synthetics don't have ANY viscosity modifiers to deteriorate. People who have done long-drain-interval oil analysis on synthetics show that they do hold their total base number (resistance to acid contamination) and wear metal values just fine at long intervals, and the viscosity actually INCREASES with age. But contamination and depletion of other additives that are already being reduced for emissions reasons (zinc, for example) are good reasons to not go much beyond a 7000 mile drain interval.
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The Mobil 1 website, for example, strikes me as being careful to say that if your car is under warranty you should follow the advice of the owner's manual or (where fitted) the oil change idiot light, but that their oils rated for various drain intervals will hold up their end of the bargain if your car allows those intervals.
The push toward longer intervals seems mostly European in origin, and is enabled by the fact that (by and large) modern engines in good condition run cleaner than old ones. Yours, although old, is from the era of modern closed-loop digital engine controls and if at 88k shouldn't be running dirty if well kept thus far, but there are things a newish car might get away with that maybe you can't. (What do your smog-check numbers look like? And what is the rate at which your engine consumes oil?)
This brings us to the question of how your particular example of your engine, subjected to your driving cycle, dirties its oil, and with what.
Some airplane people (and expensive/high-performance car people) incorporate occasional oil analysis into their maintenance observation program, but ordinary drivers of ordinary cars do not find it at all cost-effective, and where used, it is usually aimed at determining when you're coming up on the need for an overhaul, rather than when you need an oil change. I personally change it in the few to several thousand mile range, depending on the car and the driving -- when it starts to look or feel or smell dirty (yes, I sometimes get strange looks, but my engines seem to last) erring on the side of more frequent changes.
There is no substitute for getting to know *your* machine...
--Joe
PS. A quality filter is rather a lot of the battle against suspended contaminants, though it won't do a thing about chemical contaminants or particles below a certain size. I must confess to having done the occasional drain'n'fill (especially on certain cars where the filter was in a wicked location) but a premium filter (don't dismiss the idea of getting OEM ones, especially if it makes sense for you to buy a case at a time) changed with the oil is usually considered a good investment.
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The Doctor wrote:

5k-7k miles Instead of that expensive stuff, consider walmart full syn. Be prepared for seeping oil.
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The Doctor wrote:

Changing an 18 year old car over to synthetic? why? It's a great car, but if you haven't used synthetic before now, then what's the point.. If the engine has already been subjected to "de rigeur' of a standard oil base for 88K miles, then the damage that would have been prevented by using synthetic, is already done. If you want to keep the working parts in good shape, then just use a good oil/filter and change it every month..
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On Wed, 20 Aug 2008 11:21:26 -0700 (PDT), The Doctor

European cars often go 15 to 25K on synthetics and you might be able to do so but it's a bigger stretch then I would be comfortable with. I have switched all my vehicles over to Mobile 1 and a 12,000 mi or 12 month change interval. All of them had over 100K miles on them when I made the switch. One developed a valve cover leak and I had to replace the valve cover gasket. With modern engines there is no reason to be changing more often then 6000 miles/6 months when using regular oil and at least twice that on synthetic.
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How often do you change the filter?
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The podcast from WOR "The Car Doctor" has talked about this topic in depth and breath over the past summer. You can either download the podcasts from WOR or use iTunes to download most of the audio podcasts.
The Car Doctor recommends a severe use schedule of every 3000 miles. You have an older car but you will see a change in engine operation should you decide to change to a synethic oil. However your extreme use practically dictates you change the oil on a 3K interval.
There are those of the opinion that you should not change the type of oil since the engine has been on the road for an extended period using conventional motor oil. I disagree. If you like and have the budget for a CarChip or any logging codereader, drive your car with conventional oil for a week and then change to a synthetic oil.
The rwduced operating temperature of the oil and increased gas milage should be ample evidence that the synoil works better.
Adam 96 Land Rover Discovery 1 in NYC USA
wrote:

I
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Mmmm. The original poster racks up an extreme amount of use, but is that extreme use? In other words, is he delivering newspapers before work and pizzas afterward, or slogging the length of a jammed urban freeway to and from work; or just driving a long way on a fairly open highway every day? Stop-and-go driving counts as dog miles, for sure, but steady state highway cruise is a good scenario for trying an extended drain interval.
Note also that if he *keeps* driving 4000 miles a month, that odometer's going to turn over every other year. If the car is presently in a well preserved state commensurate with its low mileage, that would make me want to take care of it, including oil/filter changes as frequent as needed, which again, may be the severe, normal, or extended service interval. Those cars grow on people -- a good friend had a '91 Integra and was very sad when the second time it got stolen it didn't come back, aside from the financial reasons to keep a good economical paid-for car going as long as it makes sense to do so.
Cheers, --Joe
PS. Changing the timing belt at the interval suggested by the owner's manual is a good idea.
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On Thu, 21 Aug 2008 23:41:41 -0700 (PDT), "Adam 1996 Cherokee Sport v6

Anyone can put on a radio show. Especially on weekends. Most stations charge $50 to $250 an hour of airtime and you can say whatever you want, sell whatever you want. Weekends typically are filled with these paid shows that are little more then infomercial's. Having one of these shows does not denote any special expertise. And if said show is done by someone in the repair biz what do you suppose they are going to say? Of course they want people to come back to their shop every 3000 miles, it's another opportunity for a "50 point safety inspection" where they can sell you fan belts, filters, tire rotations, tire balance, rotate your muffler bearings, do an "injector cleaning service", clean your brakes, adjust your bands, and on and on with high profit margin, low cost "maintenance" done according to THEIR, not the Manufacturers, maintenance schedule.

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On Thu, 21 Aug 2008 00:11:31 -0500, Brent P

Same interval. I've opened them up and there's nothing there. Now if it was some 1960's carbureted fume belcher with 100K on it, it would be a different story.
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Ashton Crusher wrote:

My '66 carubreted fume belcher does just fine on 7000 mile change intervals using Shell RotellaT synthetic.
I'd worry a lot more about some modern under-designed dispose-a-car econobox with an undersized oil filter, smallest possible oil pump for lower parasitic loss, and undersized/overstressed bearings.
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I change out the mobil 1 at ~7K. not so much that I don't trust the oil or the engine running clean, but I don't trust that the filter does it's job much longer than that. The V8 is clearly a strong engine, so that's not an issue. I have thought about just replacing the filter and going with longer drain intervals. But what's the point if I am going to under there and get dirty to change the filter anyway? Might as well change the oil too.
On the winter beater I used for a couple years I did do a filter only change or two. But that car leaked enough oil that it was kind of 'self-changing'. At times I mixed the mobil 1 I drained from the '97 with some new walmart oil to top it off. Didn't do any harm.
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wrote:

Have you ever actually tried to figure out if the filter is anywhere near capacity? In the past GM used to let filters go twice as long as the oil (15,000 miles in the not so far past). I assume they had reasons for doing so. And GM always specified tiny filters (by comparison to Ford or Chrysler). I've cut numerous filters open, and usually find nothing of note. The only time I found anything disturbing was when I cut the SO's new RAV4 filter open at the first oil change. There were some significant pieces of metal in that filter, but after subsequent changes, I've found nothing. I always change my filter when I change the oil, but I am not sure it makes any sense to change the filter unless you change the oil. I've always wanted to try and figure out how much "stuff" the filter collected, but haven't come up with a good method yet. The empty filter weight is easy to measure, but it is hard to measure the weight of a used filter because they are difficult to fully drain, and because the oil soaked filter media weighs more than the dry media. I've considered. filling a new filter with oil and weighing that. Then, emptying and refilling the same filter with clean oil when removed after use and weighing it again. In theory this could give me a reasonable idea of how much "stuff" the filter has collected. Of course what I really want to know is the efficiency and flow capacity of the used filter. It is entirely possible that a used filter is actually a more efficient filter than a new filter, as long as it is not so contaminated that the bypass opens.
Ed
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No. I've just used rules of thumb of my own creation. The factory recommended interval is 5000 miles, so I take that, add about 50%... and that's the max I work with. For all I know it could last 14,000 miles but I have no real way of measuring that. For a $3 part and a total investment of ~$30 for the whole oil change, I play it safe.

Same here. If I had some way where I could *know* how the filter flowed and what it filtered out after X miles I would trust it more. Instead it's a 3 dollar part that was supposed to be changed every 5000 miles. I figure there is a generous factor of safety involved, but is it 50% more? 100%? 500%? No clue.
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wrote:

But in Europe, the same filter (with possibly different oil) is allowed to go 10k miles.....
Ed
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In my case I don't think Ford offers the 4.6L V8 or the FL820S filter in europe. (not that some don't get there anyway)
That said, each model should be checked to make sure it really is the same filter being used in europe. If it is, that would be sticking with OEM filters. (wouldn't trust another brand not to have shot for the US reccomended service interval) I would be comfortable with such a long interval in that case.
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For a $3 part and a total

Darn right. A three dollar investment for a three thousand dollar engine job is a no brainer
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