Using detergent oil in an old engine that has only used non-detergent oil might not be a good idea. Bad things that have settled out might be brought back into suspension.
What type of oil has been used in the engine?
Do you have an oil filter on the engine?
Nonetheless, which Shop Manual suggests higher weight oils? Higher than 30 weight?
Some of us use higher weight (multi-viscosity) oil in an attempt to raise the oil pressure (30W50, for example).
Here are the oil weights recommended in the first year of the high-pressure 235 truck engine.
