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"If the original engine designer was clever enough to realize that the film strength of a certain viscosity oil "
Good answer 40 year ago but, like Bohr's Atom, a bit behind the times. As it was explained to me, by an engineering dean who made his income from a long list of combustion-related patents (he gets paid a bit every time a diesel-electric locomotive is built, among other things), what counts is not simply viscosity nor "film" lubrication. But, rather there is thick-film lubricaiton, and thin-film lubrication. And in the case of journal bearings, thick-film lubrication only counts when there is still oil flooding the bearings, i.e. during a hot restart. Once the oil has drained--and it must drain unless you keep the bearings immersed and foam the oil during regular operation--all that is left is a VERY THIN FILM of oil. At that point "thin film lubrication" is what counts, not viscosity.
There are oils, mainly synthetic oils with expensive additives, that literally harden up and turn into a sheet of glass when they are "struck" with a heavy load in the thin-film scenario. With these oils, or with this type of property, when your bearings start to slam around (and they always literally slam around during startup, before there is a thick film) they slam into a thin film of oil, which promptly acts like a sheet of glass, lubricating and protecting far better than any conventional oil could.
SAE ratings? all well and good, but I'm reminded of the Army commercail that showed a Bubba announcing "Ahs could hardly change the earl in mah car, but now the Army dun taught me to maintayn this heayh Apache helicopter!" (Yeah, well, remind me never to stand under one he's worked on while it is aloft. )
Even the SAE's own web site will tell you that the oil rating (i.e. SD, SF, SM) says things that can be more important that the viscosity number. Viscosity counts--but no more than any one card in a winning poker hand.
"dont run hell out of the engine until its fully warmed up. " And for that we can thank sailors, in particular the United States Navy. Until they wrote a procurement specification for cars/trucks back in the 1940's, there was no standard for warm-up times. Then someone decided it would be nice if the targets could get moved around a bit more quickly when under attack. IIRC the original spec called for full and operational oil pressure within 20 seconds of start-up. Debates over heat loads, head gaskets, and other reasons to wait ten minutes can begin after that.
Last edited by hellosailor; 09-15-2009 at 10:33 AM.