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If you look at a viscosity curve vs temp on oils, a straight weight oil will solidify at a certain low temp and flow like gasoline at a certain high temp based upon it's "weight". Higher the number the higher the curve shifts up in viscosity (thickness vs temp). The thing oil companies are trying to achieve with multiviscosities is to alter the oil's drastic viscosity change so that you can lower the temp where the oil solidifies and raise the temp where it runs. The thicker the oil the higher the oil pressure. Hence when oil is cold it is thick and you get high pressure. When it gets hot the difference is true. Multiviscosity oils flatten out this curve and keep it thinner cold and thicker hot. Also, as mentioned, your engine bearings open up as temp goes up and they aggrivate the oil pressure problem. I ran an experiment on a MF 35 with a Perkins 3 cyl diesel. I didn't like the oil pressure variance from the time I started it till I had finished using it. It had a lot of wear on the engine and obviously the big change wasn't all the oil's fault. I put in 15w-40 diesel oil and straight 40w. Obviously the curve was higher on the straight weight oil and I had higher oil pressure the whole time. Problem is, this winter the 40w will be jello and I willl have to change to the thinner oil. That means two seasonal changes which I am not going to do. So I returned the multiviscosity and tolerated the slightly lower oil pressure. Mark
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