Originally Posted By: semaj281
Does PYB SN 5w30 still shear to a 20w after a few thousand miles of use? If OP's engine calls for a 30w and sees autocross duty, he might be better off with something that has a lower NOACK, like a 10w30 or 5w30 synthetic.
9.3 cSt is the SAE xW-20/xW-30 viscosity border. Virtually all 5w30 synthetic and conventional oils shear to around 9.0 cSt after use. xW-20 oils usually shear to around 8.0 after use. Synthetic oils are slightly better in this respect but not much -- only by a few tenths of a cSt. So, the difference between xW-20 and xW-30 after shearing (after being used) is usually less than 1.5 cSt, which is about 15%. Would that give you a 15% better safety margin on wear protection? It's hard to say and it probably depends on the engine.
Synthetic and conventional oils perform virtually the same when new. The difference happens with used oil. Conventional oils deteriorate faster in use and tend to leave more deposits and sludge when used, as they cook faster (higher NOACK evaporation). If you change your oil every 6000 miles and you don't have a demanding engine (no turbo etc.), chances are that a conventional oil is OK.