This thread is a few years old, but it is clear to me that the 20 wt oil is his problem. Nearly all old diesel engines are made for some sort of 40 wt oil. Generally 15w40 for regular diesel oil. Today the synthetic multivis oils come in a wide variety of viscosities and some engines are even built for thinner oils like 30wt, but none that I am aware of recommend 20wt oils.

I thought I was in the diesel section. What I said also applies to gassers except that they usually use 30 wt oils and some newer cars are built for 20 wt oils. If you have oil pressure problems at idle when the engine is at operating temperature, try a higher viscosity oil.

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