synthetic oil for 1970 Eldorado; what grade?

Were I to use synthetic (Mobil-1) oil in a 1970 Cadillac Eldorado [500 CI, 8.2 L, 550 lb-ft, 400 hP], which grade would be best? The car has 51000 miles on it. Is used approx. 2000 miles/year presently, though hoping to take it on some lengthy (1500+ miles) trips in near future. Car sees somewhat more use in hot weather than cold, though is driven minimally every other weekend for an hour in winter also. Largely highway miles.
Would 15W-50 be the right grade? Or should I stick to the more common 10W-30? Right now I use regular (non-synthetic) 10W-40. Please advise. Thanks in advance.
snipped-for-privacy@deslab.MIT.edu

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I am a big fan of synthetic oil......but not in this case. The different chemical structure is likely to find tiny places to leak than conventional oil didn't. The different additives might stir up sludge that wasn't a problem before.
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My advice is to start using conventional oil designated for "high mileage" or "older" vehicles. Same brand that you have been using, if they have a high mileage variety. It has special additives to rejuvenate old seals. 10W40 sounds about right.
I have a 1977 GMC pickup with about 50K miles on it. Using the high mileage oil has cut my oil consumption about in half (down to an insignificant amount).

On Wed, 23 Feb 2005, Ken Abrams wrote:
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This is poor advice. The extra seal-swelling agents added to "high mileage oil" give a short-term reduction in oil consumption due to worn seals, but in the long run the swollen seals wear even more quickly and oil consumption increases.
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Incorrect. It has additives to SWELL seals. This does not "rejuvenate" them.

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Well, at least Castrol claims they have effective conditioners.
You are right that formulators used to use solvents like xylene and others to soften and swell seals, and this led to degradation. Some probably still use them.
From the Castrol MSD, it would appear they are relying on polymer technology to improve seal condition. Polymers of this type normally don't penetrate the elastomers used in seals. Only time will tell how effective, or destructive, they really are.

On Wed, 23 Feb 2005, HLS wrote:
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Yeah, well, I'll let the more trusting and gullible types pour gunk into their crankcases and run experiments at their expense. Me, when my engines require new seals, I take the absolutely radical step of...
...repairing the problem rather than masking the symptom.

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Another case where some hard data would be appreciated, isn't it, Daniel.
We would wish to assume that a company like Castrol would have performed extensive testing, but that may not be true.
Last year, I contacted a number of motor oil manufacturers/compounders asking for hard information. Got sales blurbs back. Pure BS.

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mileage"
And in a previous reply:
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Well, EXCUSE me. Obviously you are an expert on everything!
The real experts recommend using the same brand and grade of oil all the time.
My experience leads me to believe that synthetic WILL find leaks that conventional oil does not. The '95 Seville I had previously was changed over to Mobil 1 at about 30K miles. It immediately developed 3 small leaks that were not there before. Coincidence? Maybe but I think not.
On the seals subject, what you say might be valid for a vehicle that does not have appreciable seal wear or shrinkage. Assuming that the additives swell the seals and they stay that way, this makes up for excess clearance due to wear or shrinkage due to age. It is true that seals that are tight will wear faster than ones that are loose and leaking. This not a bad thing.
Your "bedside manner" stinks. Everyone who has an opinion different from yours is not (necessarily) an idiot. Even if they ARE, it is rude to point it out in public. (But somehow I suspect you don't care)

Ken Abrams wrote:
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And my experience does not. I switched my 73 318 over to Mobil 1 with well over 200,000 miles on it (since its rebuild at 190,000 miles, that is). No gushers, just the same old persistent weeps that smallblock Chryslers tend to get (front and rear intake valley seals, primarily). Been running Mobil-1 in it for years now,

Steve wrote:
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Maybe an odd question, but I'll ask anyway... how does Mobil 1 act in an engine with the old style felt front main seals? I'd like to run Mobil 1 in my Studebakers but they leak enough with Rotella as it is. Haven't gotten around to getting a neoprene seal installed in a timing cover yet, although if I could ever stick with a car for more than a couple years I might (I thought my '62 hardtop was a keeper, until I fell into a '55 coupe...)
nate

Nate Nagel wrote:
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Well, Mopars only had a felt REAR main. My 318 probably has neoprene on both ends since its been rebuilt. But I have run Mobil-1 in my 383 and in my 440 with rope rear seals, and they never leaked. In fact, I've never had a rear main leak period (now sure as I say that, one of them will leave me a nice puddle in the floor....)

Steve wrote:
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Heh... Studes all had neoprene rear mains and they STILL leak like sieves. In fact, if I back out of a parking space and don't see a little dot of oil, I immediately look at the oil pressure gauge :)
nate

On Wed, 23 Feb 2005, Ken Abrams wrote:
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If you don't like my posts, or don't agree with them, then I'll put in for an exemption for you so that you're no longer forced to read them.
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Oh? Which "real experts" would those be? On planet Earth, the real experts recommend using a reputable brand of an appropriate grade for prevailing conditions.
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It is a very good thing, then, that I haven't got a medical degree.
DS

What is your goal in switching to synthetic?
With a car this old (even though relatively low mileage) I don't see much reason to go to synthetic, but then I am not a staunch fan of synthetics anyhow.

Just depends on how fast you want to clean it up off the ground. The thinner oil will run out of all the seals really fast, the thicker will only run out really fast when it gets hot, it will slow down when cold.
The trouble is synthetic will clean up all the sludge build up on all the seals and gaskets and they 'will' start leaking on something that old because this build up helps seal, so unless you just totally rebuilt the engine with all new seals and gaskets, I certainly would not recommend switching to synthetic.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
"F. Baker" wrote:
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On Wed, 23 Feb 2005, Mike Romain wrote:
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Mike, shame on you. I know for a fact you're not ignorant enough to believe this. This bullshit has been floating around since synthetic oil first hit the market. Like most good piles of bullshit, it has at its core a kernel of truth: The first synthetic oils available were 5W20 and 5W30, which were very much thinner than the conventional oils widely used at the time (1970s). So, existing small leaks leaked more of the thinner oil, and the myth that synthetic oil causes or worsens leaks was born. Over the years, handwavers added all kinds of nonsense about synthetic oil dissolving sludge that helps the engine seal, etc.
What you said above is just plain incorrect. Engines that leak conventional oil will leak synthetic oil. Engines that don't leak conventional oil will not leak synthetic oil. That's sound theory, backed up with years of personal experience, and it trumps handwaving and tongue-clucking and pseudoscientific baloney.
To the original poster: You should have no trouble using Mobil-1 in your Cadillac, though it's hardly necessary. Even the cheapest conventional engine oil available today is VASTLY, HUGELY better than even the best engine oil available when your car was new. If you do go ahead and use Mobil-1, 10w30 or 15w40 would doubtless be fine.
DS

I guess I just haven't caught up with the times then. The only experiences I have with the overpriced junk was to watch it turn engines into pincushions back a few years ago.
Normal gasket seeps or wet spots that weren't enough to ever drip turned into taps running bad.
If the OP does go for it, I sure would appreciate a follow up in a month to see what happens....
The only time I dared to use it in any of my vehicles was just after a rebuild of my t-case and tranny seals and then I only used a semi-synthetic gear oil because they didn't sell any 'real' gear oil at the local auto stores without the friction modifier that my T-5 sure doesn't require.
Mike
"Daniel J. Stern" wrote:
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On Wed, 23 Feb 2005, Mike Romain wrote:
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Uh-huh. Mobil-1 is "overpriced junk", and carburetors react well to hammerdrills, and all the other imaginative little stories you tell, Mike.

"Daniel J. Stern" wrote:
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Hey, I just thought you were an occasional mouthpiece, not an outright liar Daniel, what gives?
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Daniel J. Stern wrote:
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That's the traditional Amsoil line about cleaning up sludge.
Really though, polyalphaolefin tends to harden seals. So assorted ingredients are added to improve seal compatibility, like an ester base. Newer seal materials should be more compatible with PAO.
I believe the initial Mobil 1 formula (PAO only) used to have horrendous seal compatibility problems. I wouldn't worry about the current Mobil 1 with seals made in the past 15 years.
Now if you're talking about the original seals of an early 70's Eldo, that might not be a good idea. They might not take to kindly to being exposed to PAO.

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I think it is a false assumption to believe that every engine of age is loaded up with sludge.....let alone believing that synthetic additives somehow have the power to dislodge any sludge that mineral oil additives had not.
I sold an Astro van with a 4.3 to my cousin, who then decided to rebuild the engine since it had over 205,000 miles on it.
When he pulled the rocker covers, he called me and asked what type of oil I had been running.
I told him that I always ran whatever API-rated oil was on sale, and I changed oil and filter at 3,000- 3,500 miles...and that the oil that was in the van when I sold it to him was $9.99-per-case, 10W-40, "Certified" brand oil from Sam's Club......
.....why was he asking?
He said you could run a white glove over the top of the head - under the rocker cover.....it was THAT clean.
Since the engine was NOT burning any measurable amount of oil, he put on a new rocker cover gasket, and is still running the van approaching a quarter-million miles - albeit measured in Canadian kilometres these days - using the same maintenance schedule.
So, the presence of sludge isn't necessarily a foregone conclusion for engines over 50K, 100K .....or even 200 K for that matter.