Been using 20-50 since the 80's in high performance motors to make them last. 5-30 in the light duty customers cars because they tend to blast away while the engine is still cold.
Recently I began obtaining Mobil 1 0-40 direct from my parts supplier for my cars and hi-po customers cars. This is what MB,BMW, Porche etc are requiring in Europe where they are more concerned with high output motors lasting rather than the CAFE mania here in the US.
If anyone is more concerned about gaining 2.5% MPG at the expense of engine life 5-30 is the way to go, otherwise get an oil that does not thin to 7 cp at 230 deg F as 30 wt does.
For anyone that is interested, I have temp vs viscosity curves for Mobil 1 and Mobil drive clean multi vis oils in pdf format. Looking at the curves between 60 deg F and 230 deg F show about a ten to one viscosity change over this temp range. Temp change has a greater effect on viscosity than choosing a different SAE grade.
For those that are worried that a 40 wt oil is to thick for our modern Contours- the curves indicate that putting in a lower temp thermostat or an oil cooler to get a 25 deg drop in running oil temp is like going up a grade in viscosity!
Conversly, beating on the car in summer and pushing the oil temp up effectivly changes your oil viscosity from 30 to what a 20 wt oil would be at the nominal 100 deg C.
I am not surprised to hear about the occassional spun bearing on these boards when the prevailing wisdom is to follow the CAFE driven recommendation for 5-30.
My choice for obtaining the 150k + trouble free miles in my "actively" driven vehicles is either the 0-40 syn or 10-40 dino fluid.
