Mercedes and VW
 

Why is it that Mercedes, Peugeot and Volkswagen recommend using 10W40, 15W50, 20W50 oil and not using 5W30 and 10W30,
but Everybody else, including BMW, all the japanese cars, etc recommend using the opposite. They never recommend putting such a heavy oil in.
I started using 15W50 in the mercedes and this still bugs me. I always thought heavier oils are bad for startup and make the engine sluggish. I really really want to put 5W30 in the mercedes but everyone tells me it is not recommended for this car but nobody can give me an explanation why.

1991 500SL

Maybe because engines run hotter these days.

VW currently recommends 5W-40 energy conserving (read as synthetic) with the alternative of 5W-30 if 5W-40 not available. BMW is recommending 10W-60 in M3s. M-B is recommending 0W-40 synthetic for the latest engines. For older cars such as your '91, you should select oil according to the "lowest air temperature expected" by using the factory chart. 5W-30 and 10W-30 are certainly OK.

I would prefer using 0W40 but the dealership said they recommend using 15W50 in my car for it to run properly. From the next 9 months we don't expect any sub zero temperatures so I should be okay with 15W50, but doesnt 15W50 increase wear over a 5W30?

Either is fine. I think you are worrying about this entirely too much. 15W-50 is in accordance with the chart at any air temperature above +5 degF. 0W-40 has no temperature restriction. However, when your '91 500SL was designed and built, 0W-40 wasn't even available. It is only recently, with the implementation of the FSS system in 1998 (with the M112 V-6 and M113 V-8), and the extended service interval, that synthetics and 0W oils became a recommedation. In fact, all the latest recommendations have more to do with trying to extend the interval to more than 10,000 miles rather than any wear issues.

The reason that X-30 weight oils are not recommended is that they provide lower oil pressure at hot temperatures. The reason American and Japanese car companies recommend them is that it allows them to gain a tenth or so MPG when they report their CAFE averages to the government. These oils provide less pressure, and conversely less drag and more fuel efficiency at the expense of engine life.

The first number is what matters for cold start wear. The weights are determined according to the kinematic viscosity at a certain temperature. I use 0W-40 Mobil 1. It provides a kinematic viscosity of about 15 cSt (which is in line with any 40 weight oil) at 212F yet still pours at -60F.

If you want to save .1 MPG and do not care about your engine, then by all means use 5W-30 in your MB engine.

Ok thanx i feel better now about using a heavy oil :)