Using 5w20 Oil

As long as both kinds of oil meet or beat Subaru's requirement of viscosity and API specification for spark-ignition engines I'm pretty sure you'll be just fine. My owners manual specifically says I could use different brands simultaneously, as long as they meet the specs. My '02 WRX requires SH or newer - check your owners manual for your Outback if you want more piece of mind. Mine prefers 5w-30 for general use, but allows the use of 10w30 and 10w40. It also says that under severe duty, SAE30, SAE40, 10w50, 20w40, or 20w50 can be used to suit the load and expected temperatures. The latest designation is SL. If the car requires SH, you can use any combination of SH(1994), SJ(1998), or SL(2001?) oils. You just couldn't use an oil that only met an older designation like SG(1987) I've always used the recommended oil. I moved from Boston, MA to Phoenix, AZ nearly a month ago (it's 110F today) and am switching from fossil oil to Mobil1, still in the 5w30 weight.

- Byron

If I can trust my dealer he said NOT to use anything heavier than

10W30 because you don't get the proper lubrication. I.e., switching to 90W gear lube certainly wouldn't improve things, eh? ;-) Username munged by FixNews

Wow that is interesting since the difference between 10w30 and 10w40 is small in terms of viscosity.

What car do you have? My Impreza manual says I can run 20w50 if I want, but

5w30 will yield best fuel economy.

Wouldn't the difference between 10W30 and 10W40 be 25% difference? That's quite a bit. And for the worst temperature condition.

The higher number represents the viscosity at cold temperatures when the engine need lubrication to help prevent piston slap, etc.

D>

Username munged by FixNews

I ran 20w50 all the time when I lived in Oregon because of the moderate climate. It's too cold here in the winter to run that thick. Engine won't even turn over without a block heater.

It does get pretty hot here in the summer so I run a thicker oil then because it will stay thicker under higher temps.

Hi, I think you're misinformed. No mixing! period. Tony

You might want to tell the engineers at Subaru that they are also wrong, because my info came right from my owners manual (page 11-13). A mechanic I know who has built hundreds of high performance engines also agrees with them and me - meet or beat the API service rating, and keep the viscosity rating at one of Subaru's recommended levels for the load and temperature range your car will see.

Mobil1 oil also states that it can be freely mixed in any proportions with conventional oil.

Where does your information come from? Can you post a link? Or explain why you think Subaru is wrong?

Hi Byron,

I'm moving from TN to Phoenix in two weeks and am curious if dealers there recommend a particular weight for that hot climate. I was going to change oil in my ForesterS this week but decided to wait and introduce myself to a Phoenix dealer, since I will be trailer towing the Forester behind the rental truck. I only find one Phoenix dealer on My.Subaru.com, but they may have to 'special qualify' or 'pay' to get a listing. My dealer here isn't listed either but one 30 miles down the freeway is.

We'll be house shopping in Sun City West if we survive packing and loading. :-)

BoB

I remember an NG thread about mixing oils. Someone with excellent credentials (oil/fuel additive developer and engine lab director at Exxon) claimed that some detergent additives mixed together could cause (maybe a 50/50 mix) of two different 5W-30 oils to thicken out to a 15W-40. I don't think this is that big a deal if you're changing oil.

I think now there's probably far more similarities than differences in additive packages these days.

Not really. The number 30/40/50/etc represents a range of viscosity that meets the requirement. A 10W-30 oil could be right at the upper limit of just under 12.5 cSt at 100°C. A 10W-40 could be right at the lower limit of 12.5. These probably aren't typical.

These are the viscosities (in cSt at 100°C) for Chevron Supreme motor oil (from their datasheet):

5W-20: 8.0 5W-30: 10.8 10W-30: 10.8 10W-40: 14.8 20W-50: 19.0 30 wt: 10.3 40 wt: 14.0

I also found it interesting that their 30 wt oil is thinner at 40°C (104°F) than their 10W-40 or 20W-50. If you're running it in a desert climate, it sounds like 30 wt could be a better choice than

10W-30 for cold engine starts.

Besides that - I don't think that viscosity can really be quantified as X% thicker. It's just a number that might not be linear with different scales.

But it's only a range. Not all 10W-30 motor oils are of the same thickness.

So then if you use 2 different weights from the same brand it should work just fine.

But also take into consideration that a "cold start" in a hot climate is at a warmer temp than a cold start in a moderate or colder climate.

Yes. I only used that example because the datasheets I had listed viscosity at 40°C (a hot climate) and 100°C (near operating temps). I still found it odd that 30 wt oil would be thinner at 40°C than their

10W-30.

I also have the feeling that what's currently called a "straight weight" oil might have been considered a multiweight oil in the past. The manufacturing process for motor oil is likely far different than even a decade ago.

Perhaps. I once called the Mobil 800 number and asked if it was OK to mix equal part of Mobil 1 0W-30 and 5W-30. It was what I had on hand, and the answer was yes.

However, motor oil makers modify their base oil and additive packages all the time. Castrol states just that on their label. I've seen the same brand/weight with noticeable color/odor differences; I bought them at the same time. I couldn't imagine a radical change in additive package that would lead to thickening though. I'd think an oil maker would test for compatibility with their previous formulations.

I found the original post I referred to:

That's true. My dad keeps telling me those stories, "I remember when there was only one type of oil you could buy back before all the multiweights were available......."

My dad was stationed in the army in Alaska and he also told me stories about motor oil in the winter time up there. :)

Don't know if this moves the thread forward, but here goes. Viscosity is a measurement of what we generally conceive as the thickness of a fluid. Pudding has a much higher viscosity than beer. Jello is a pretty normal fluid when we're taling about viscosity. When the Jello is just made, it's hot and thin. After time in the 'fridge, it gets rubbery thick.

Liquid viscosities change because they are affected by temperature and pressure. The molecules have more or less energy and pack less or more tightly as the temperature goes up. .... That's in liquids which obey the rules of Thermodynamics Newton put together in the 18th century.

NON-Newtonian fluids DON'T FOLLOW THE RULES (those pesky creatures). They keep the same viscosities at different temperatures and pressures. Ball point pen ink is an example of a pressure-dependent non-Newtonian liquid. In the pen, it's REALLY thick. At the moment of shear, under the pressure of passing over the ball and onto the paper, it's viscosity collapses and it flows onto the paper where it reqains its earlier thickness and doesn't run all over everything.

Modern motor lubricating oil is made to keep the same thickness despite dramatic changes in temperature AND pressure. Since the Society of Automotive Engineers is loath to change anything they've been using for awhile (consider the metric system of measurement) they created a way of defining motor oil viscosity which encompassed the modern keep-the-same-thickness-across-a-temperature-range "multi-viscosity oil"

10W-40 or 5W-30 really means the viscosity STAYS THE SAME at different temperatures and pressures. You can still find 30W and 40W oils in some stores. People used to say,"Thirty weight oil" or "Forty weight oil" when refering to these fixed viscosity lubricants.

If you expect to operate your vehicle in high temperatures all the time, a heavier range of oil will protect the engine as it first starts running, but not break down in the heat. If you're heading to the polar cold, thinner oil won't turn to jelly when it's cold, but still protect the engine when it's at operating temps. You still need the lubricating capability, so the ranged viscosity keeps the engine protected across the temperature and pressure ranges.

We have an extreme range of temperatures where I live. I change my own oil and I notice substantial differences in the viscosity of the oil both out of the jug and out of the engine when I change it based on the temperature outside. It pours easier when it's hot and not as easy when it is cold. If it is cold enough, it pours even slower.

That's backwards:

When cold, 10w40 acts like a 10w When hot, 10w40 acts like a 40w

The question I have is protection from start-up wear and in the high speed bearings of a turbocharger?

One of the big oil companies (Castrol, I think) is making a big deal about its special polymers thant stay on the metal and lube it during those crucial first seconds after sstartup. Anyone know how much is truth and how much is marketing?

Also, the high speeds and extreme temperatures in a turbo would seem to demand a unique oil formulation. Anyone know if any of today oils or synthetics are better for turbos than others?

Thanks,

Philip

I remember (decades ago) people in the Montreal Alfa club liked the oil additive "MolySlip" (with molybdenum disulphide ISTR?) This worked sort of like a suspended high temperature graphite grease: slippery "plates" of molecules, which would settle on bearing surfaces and supposedly provided lubrication when there wasn't much oil in the journals. I remember using it in my 2000 GTV a couple of times, too. One of the track racers claimed it saved his engine once when he put a hole in the oil pan and didn't notice (until the smoke?). OTOH, others warn against additives that might gum up the works. Dunno. Anybody use anything like MolySlip these days?