oil viscosity grade

Not open for further replies.
Jun 16, 2003
If a auto manufacturer specifies that a 20 straight-weight, dino oil is satisfactory for an engine up to 50F, what can you infer from this guidance?

To me, this means that 20 weight oil is fine, regardless of ambient temperature, as long as the oil temperature stays below a certain maximum temperature. (That is, the manufacturer is implying that as the engine's cooling system is increasingly stressed by higher ambient temperatures, the oil temp will rise to the point where the 20 weight oil will become too thin.) So, by looking at that manufacturer's viscosity chart, is it possible to guess what "too hot" is and what minimum viscosity the manufacturer is trying to maintain?

by looking at that manufacturer's viscosity chart, is it possible to guess what "too hot" is and what minimum viscosity the manufacturer is trying to maintain?


The cooling system runs at just about a constant temperature as soon as it reaches the thermostat set point. Your engine oil is cooled mainly by air passing the oil pan and lower block. You may also have a water-cooled oil cooler, but that will be constant coolant temperature, and some oil cooling is done by heat transfer through the metal to the coolant.

Why are you asking? What's your goal?

Why are you asking? What's your goal?

This is somewhat oriented to understanding the thick/thin debate. The engine is the bmw m20 engine, and a 20w-50 oil or straight 40 weight is recommended at high temperatures. So on the one hand 20 weight oil is ok, but on the other 40 & 50 weight oil is recommended. If the engine doesn't have an oil cooler, they recommend going to the next higher grade for sustained highway travel.

The idea I had was that their recommendations must be driven by the decline in viscosity at higher oil temperatures, and thought you might be able to figure out what that 'target' viscosity was. If you knew that and had an oil temp guage, you could run the thinest oil necessary to maintain the target viscosity.

So, I'd like to know more about the thought process they might be going through when recommending 20 weight versus 40 & 50 weight oils. It seems clear that 20 weight is adequate, but it isn't clear when&why 20 weight becomes inadequate.
Ray, when is it adequate?...basically when the manual says so.
I have 89 325is (same motor) and I have an oil temp quage. With Red Line 10w30, when ambient is 90F the oil temp is approx 195F.
So, yes if I wanted same viscosity in winter as 10w40 in summer,I could run RL 10w30. And not worry because it's basically same as the 10w40 is in summer.
In fact I did analyze it through late winter into summer and it was fine. I'll post it one of these days.
Changing visc w/season would interfere with change intervals. So I'll just run 10w40 year round from now on.

Check this out too:

Our engines don't seem to experience as much OT change as that though. Probably because of the oil cooler. And below say 50F or so, my OT is NEVER above coolant temp contrarty to that article. (unless I beat on it)
BTW Ray, the "when the manual says so" was for the straight grades. BMW multigrade recommendations are very conservative. If you take the stability of the oil you are using into account you can go thinner than they say. i.e. if you use a completely stable multigrade oil (one immediatly comes to mind) you can use the second number and refer to the straight grade area of the chart.

when ambient is
If the oil temp is about 160F in cool temperatures, then the kinematic viscosity of a 20 weight oil would be about 14 centistokes. I have heard that when pushed hard, this engine gets oil temps up to about 230F. To maintain 14 centistokes at that temp you would need a 40 or 50 weight oil. As it turns out, this is the recommended oil weight for hot ambient temperatures.

I wonder if there is a 'threshold' for kinematic viscosity below which the engine would tend to wear quickly. (Is there a relationship that relates kinematic viscosity and wear?) Does this have any relationship to the 'knee' the the HT/HS wear chart (at ~2.6 HT/HS).
Not open for further replies.