lrd@drusd.UUCP (DuBroffLR) (12/12/84)
Sorry about this folks, but I'm getting a little tired of seeing all the self-styled experts spouting off with their own notions of "facts." This topic has gone on and on, and lots of misinformation has been presented with authoritative style. DON'T BELIEVE A WORD OF IT! A manual transmission may, depending on the manufacturer's recommendations, use hypoid gear oil, motor oil, or even (gasp) Automatic Transmission Fluid (ATF)! Further, neither Chilton's nor your owner's manual (as suggested by one net.poster) may be right. If in doubt, check directly with a representative of the manufacturer. Case in point: The owner's manual for by Mercedes-Benz 240D indicates that 10W motor oil should be used for the four-speed manual. This is confirmed by Chiltons and several other similar publications. The service department at a Mercedes-Benz dealer filled the manual transmission with ATF. When I noticed this on the bill, I questioned them; they showed me a service bulletin from the factory, indicating that this was now the recommended lubricant for my manual transmission. After another major service (50,000 miles later) by another service organization, I noticed stiff shifting. I brought the car back and questioned them. They had filled the transmission with 10W oil, as specified in all the "authoritative" publications (they were not aware of the factory service bulletin). After some heated discussion, (including the notion of a stop payment order for the check), they agreed to drain, flush, and refill the transmission with ATF, at no additional charge to me. The Automatic Transmission Fluid is evidently doing the proper job in this manual transmission; with over 115,000 miles on the car, driven hard in weather ranging from sub-zero to over 100 degrees, the transmission has never required attention other than the recommended lubricant replacement. Shifting action is as light and positive today as it was when I drove the car out of the factory door, in 1978. And, being an underpowered (62 horses) diesel in a full-sized car, the transmission gets a lot of use, especially here in the Colorado mountains! Moral of the story: The "experts" may not have the facts. "Chiltons" or other similar publications may not have the facts. Your owner's manual may not have the facts. Your trustworthy mechanic may not have the facts. If in doubt, check with the source -- the manufacturer's tech rep!
paulhus@euclid.DEC (Chris Paulhus 223-6871 MLO8-3/T13) (12/12/84)
A few comments on transmission lubricants: Originally (say 1950's and 1960's) manual transmissions used an EP rated oil, the EP standing for resistance to extreme pressure which occurs at the gear tooth contact point. In the late '70's, GM decided that stocking two fluids, an automatic transmission fluid (ATF) and an EP fluid was too much trouble for their dealers and redesigned their manual transmissions to allow ATF to function as a lube. ATF also has viscosity improvers that allow it to function better at low temperatures, similar to multi-grade motor oils. The EP rated oil was given a viscosity range rating higher than motor oils (despite having the same real viscosity range : a 90 EP oil has about the same viscosity as a 40 weight single grade motor oil!) so the two products wouldn't be confused (like us acoustics types are using decibels for sound pressure and Bels for sound power). So, a 10W-40 motor oil is similar to the 90 weight EP transmission lube of old, less the extreme pressure additives (but the new SE and SF motor oils do pretty good on this score), but with superior low temperature performance. Viscosity improvers (VIs) are long chained polymeric (sp?) molecules that coil up when cold and add nothing to the base stock's viscosity. As they warm up, they uncoil into thread-like strands. This effectively increases the viscosity of the mixture (base stock plus viscosity improvers). In highly stressed engines (high contact pressures, lots of shear stress at the lubricated surfaces - typical of higher rpm, smaller engines) there was a tendency for the long thread-like VIs to be sheared or cut, decreasing their effectiveness. After 20,000 miles your 10W-40 ended up with a bunch of chopped up VIs in a base stock of 10W oil, which didn't provide adequate protection at 180 deg. F (or more in VWs and such!). (Remember, the 40 weight is just the effect of the VIs on the base 10W stock.) This situation is better now than it was 20 years ago, but users of multi-grade motor oils - especially high rpm users - should be cautioned to obey the frequent drain intervals recommended. (The other reason for frequent drain is the breakdown of the other neat additives in the oil - anti-rust, anti-foam, anti-acid, etc. This is particularly important if the oil doesn't get up to temperature and volatize off the acids and other bad stuff.) Another 2 cents worth from: N. Chris Paulhus DEC-Maynard decwrl!euclid!paulhus (617) 493-6871