What is the correct oil viscosity?
 

According to my owners manual, the correct oil viscosity for my car is 20W-50. I assume that this is the right oil when the engine is new, but does the optimal oil change after 150,000 miles?

Oil viscosity recommendations stay the same. I would stick with the correct viscosity for your ambient air temperatures.

This has come up before, which makes me think that some USA owners handbooks do not have the chart, that shows what you are asking.

If you do not have the chart, the local MB Dealer, show be able to give you one.

Regards

Col Tigwell Downunder

I have the chart for viscosity vs temperature, but not one for viscosity vs overall milage. I'll take the advice from above and stay with the 20w-50.

My MB dealer put in Mobil 5W30 about a month ago in my 1992 300E, which doesn't match the temp chart at all for a midwest summer.

What's up with that?

It's my understanding that heavier oils (10-40/20-50) are really just lighter oils with a "thickening agent" (similar to STP) added to them. So they're just making your oil pump work harder, but not lubricating any better.

Jeff Pierce

Thicker oils have a higher viscosity over the entire temp range. They also have a higher film strength and flash point. So, they do lubricate better.

So, is Mobil 5W30 for the summer bogus? Should I switch to 20W50?

The viscosity recommendations have changed from when my owner's manual was printed to the most recent list of "approved flluids" from MB. That list can be obtained from MBUSA (1-800-FOR-MERC) for free or a very small charge.

I have read on the Ritter/Easley site that Mobil 1 5W-30 is no longer a factory approved oil -- not sure why. Mobil 1 15W-50, 0W-40, 10W-40, etc ARE approved.

I used to use Mobil 1 5W-30 but have switched to 15W-50 year round.

There is no change is viscousity, based on mileage. According to the ".......service products" list, 20W-50 is only good for temps. ranging 23 deg. and up. Since you live in Mass., I know it can get below that in the winter. 15W-50 would be good to as low as 5 deg. This of course conflicts with my TD's owmer's manual, which say 15W-50 is only good down to 23 deg. and the forementioned 20W-50 is only good down to 32 deg. Now, it can be argued until the cows come home that over the years since our manuals were printed, that oils have improved. The "service list" does not differentiate between synthetic and conventional oil, when it comes to recommended weights. Some also argue that a "thinner" synthetic is as good as a thicker conventional oil. For
Mass., I would go with 15W-50.
As far as Mobil 1 5W-30 goes, I don't think it ever was an approved oil. In Europe all engines take at least a xxW-40 oil, which has a classification of A3. Mobil 1 5W-30 is rated A1. The ONLY Mobil 1 sold here that meets this standard is 15W-50 and if you can find it 0W-40. According to the "list" of the 11 synthetic oils approved, only 2 are 5W-30, Castrol TXT Softec Plus and Shell Helix Ultra. They must be sold only in Europe. All the other oils listed are 0W-40, 5W-40, 5W-50, 10W-40 and 15W-50. Also, alot times dealerships put in whatever oil they get the best price on, like the dealer mentioned that put in Mobil 1 5W-30. You could challenge that dealer about their using a non-approved oil in your car.