You are an engineer, aren't you? I can smell them -- my son is one.
On 6/26/06, Peter Frederick <[EMAIL PROTECTED]> wrote:
Actually, viscosity ratings are more complicated. In the old days, you have an oil of a particular viscosity (actually flow rate at a certain temperature). Petroleum based oils have a significant change of viscosity with change in temperature character -- when hot, they get VERY thin. When cold, of course, they aquire the flow characteristics of molassas, what ever the room temperature viscosity. The solution was to add materials that didn't change viscosity so much, so that an oil mixture would have "multi-viscosity" characteristics -- that is, flow like say a 10 wt oil cold and a 30 wt oil hot. The rating is cold viscosity rating number first, followed by hot viscosity rating, so a 10W-40 oil flows like a single rating 10W oil cold and like a single rating 40W hot. The base fluid used in synthetic oils doesn't change viscosity with temperature nearly as much as petroleum base oils, and in fact is fairly hard to rate using methods for petroleum base oils. 0W-30 synthetic oil pours like cold diesel fuel, but I'd not want to use diesel fuel as an engine oil -- wouldn't stay in the engine! The smaller molecules will likely burn off first, since they are more volatile, but I suspect there really aren't that many in synthetic oil. The majority of oil consumption in a good condition engine is burnoff from the cylinder walls as they are exposed during combustion, along with whatever amount burns off the valves from clearance on the stems. Synthetics are very resistant to oxidation (remember the frying pan on the grill ads?) so they don't burn off the cylinder walls, and the molecules are fairly large so they won't evaporate. No modern oil has components light enough to actually vaporize, I would venture -- they would also carbonize too easily, leading to excessive sludge. What the viscosity of a mixture is would be hard to determine without actually testing it, and I suspect what you would have would be xxW-50 -- the viscosity "extenders" would work their magic more at the hot end than the cold end. Fun stuff, eh? No wonder oil threads go on forever! Peter _______________________________________ http://www.okiebenz.com For new parts see official list sponsor: http://www.buymbparts.com/ For used parts email [EMAIL PROTECTED] To Unsubscribe or change delivery options go to: http://okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
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