Originally Posted By: zeng
Plain bearings with abrasion scratches over times,even if bearing clearances hasn't enlarged, would require higher operating viscosity at similar conditions of engine rpm and load than an original bearings.........
just to maintain similar MOFT, and by extension film strength.
It makes sense to up a grade or two for used bearings ..... and used interfaces at cylinder liners and cams.
CORRECT!
One easy way to figure out if the engine is worn is to keep track of the oil consumption figures. In general terms I would only move up one grade from OEM spec, BUT I would find out what that spec was for use outside the USA, as the one quoted from a US site might be biased by fuel economy regulations.
It also helps with an old worn engine to use an oil with more anti wear additives and detergents, although I would only switch to an HM oil when the oil consumption increases.
It's also very important to select an OCI that matches the condition and use of the engine, as older engines tend to contaminate their oil more. The best way to figure out what the OCI should be for an older engine, is to get a used oil analysis (UOA) done.