On 8/9/19 10:00 AM, Mitch Haley via Mercedes wrote:
You don't understand or don't trust the winter viscosity rating.
There is NOTHING WRONG with 0W or 5W oil in our old indirect injection engines.
Let's start with 15W50 M1.
At 100°C, its viscosity is 18 cSt.
If we could make an oil that was 18 cSt at 100C and 18 cSt at 0C, we'd have the
perfect oil.
But, alas, at 40°, its viscosity is 125, and at -20C, it's over 3000.
Now let's look at 5W30 M1.
At 100C, it's 11.0, thinner than we want, but any thicker and they can't rate
it as SAE30.
At 40C, its 62, which is thicker than we want, but thinner than 15W50.
So, we see that the best way to choose is to follow the MBZ manual, with a W
rating acceptable at the lowest temp we'll park the car in and the 2nd number
acceptable at the warmest temp we'll drive in. For an OM-617, I believe the big
number is going to be 40-50 in all but the freezing weather, and 20W is
acceptable above 0°C IIRC (check your manual, my memory isn't perfect).
The historical fear of low W-rated oil is well founded.
If I used 1979 era 10W40 in my 116.120 and drove through Arizona in July at
90mph, it might turn into 10W10, or at best 10W15. 10W40 oils were famous for
thermal breakdown of the VI improvers, expecially in motorcycles, lawn movers,
and turbocharged cars. In 1979, M1 5W30 (the only grade available) was far
superior to any dino 10W40 in a 617.950, because it stayed SAE30 at high temp.
By 1986, we could buy M1 15W50 with the red stripe on the can.
On August 8, 2019 at 10:18 PM Curley McLain via Mercedes
<mercedes@okiebenz.com> wrote:
Zackly! The OM 616/617 trace their original design to the OM 621, which
first appeared in ~1958. The actual design work is ~70 years old. No
5w-x is appropriate for these engines. It is not appropriate, and not
approved. Original oils were single weight. 10W-40 was approved in
the late OM621s, (wnen it became available) and then for the OM 615, 616
and 617. For engines with in excess of 200k kliks, (120K miles) 15w-50
works well.
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