Loose bearings can stand 20w50, rebuilt engines should have a 10-30 or 10-40, straight 30 or straight 40 if it gets really hot.
Clean engines can run any type of oil you want, the thinner oils will seep out seals quicker then thicker oils. Dirty old engines should have straight weight, non-detergent oils.
New formula oils have a high detergent level to move as much dirt and particles as it can, as fast as it can to the filters. This is how they get by with 5000+ mile oil changes, that and the engines don't contaminate the oil like our old carbureted engines do. A non detergent oil, will not clean, so the dirt and sludge stays were it is, stuck to everything, but not floating around in the oil. If you add detergent oil, you risk the chance of all that sludge getting loose and flowing with the oil, which could possibly hurt the bearings. Full flow oil filtration would clean it out, but partial or no filtration is asking for trouble.
