by 1972nail » Mon Dec 29, 2014 6:01 pm
Go for a quality 20/50 oil. If you let the engine idle to warm it up the water temp gauge may show normal but the oil temp may have risen way above normal operating temp and get extremely thin. This is a not an uncommon problem with the slant 4 especially with old oil, a low oil level and reasonable miles on the clock.
If the oil does reach this 'shear temperature' enough times it's performance drops off dramatically at lower temperatures too and the critical shear temp also drops way down as well. This results in lowering oil pressures as time goes on.
So, to cut a long story short, change the oil for a good one, only let it idle at operating water temp for a short time and if possible, take it for a drive to warm it up and get cooling air around the sump at the same time. A large capacity oil filter also helps with the cooling a bit as well.
If your car is used in traffic a lot consider the addition of an oil cooler as a worthwhile safety precaution.
Editor of the VVOC Magazine ReVival
"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.