Motor oil is mostly used to lubricate an engine in a dynamic mode, meaning that two parts are separated by a film of oil and one moves in relationship to the other. The parts themselves never actually touch each other, only the oil. Shifting is different in that it uses static lubrication where two (or more) pieces of metal actually touch and drag against each other. The parts are still covered in a film of oil but there is no speed to have the parts use the oil dynamically; think of it as one part 'oil skiing' on a film of oil rather than sliding against its mating part.
Shifting involves sliding parts against each other and uses a mechanism called boundary lubrication of the oil. I am not sure what component(s) of motor oil contribute to smooth or notchy shifting but they do appear to be the materials used for static, boundary lubrication; materials such as ZDDP, moly, sulphur, chlorine, and perhaps others. Motor oil generally uses only ZDDP and moly as boundary lubes and generally only ZDDP. I would be tempted to say that the reduction of ZDDP in motor oil over the last couple of years is what is causing notchy shifting because bringing the ZDDP level up results in very noticeably shifting. But I have also found Valvoline and Pennzoil auto grade motor oils to shift extremely smoothly, at least when they are fresh and clean, and I do not believe either one of those contain any more ZDDP than Rotella. ?? There must be another compound(s) used that cause the difference in addition too, or in lieu of, ZDDP.
Another odd thing I have noticed is that the auto oils I have tried start out great regarding shifting ease and smoothness but deteriorate fairly rapidly so that by the time the oil has something like 2K miles on it the shifting is again rough. But Rotella 15W-40 with the zinc level raised seems to shift well and about the same as new when the oil has more than 3K miles on it.
I have been tempted to add some zinc to oil that has accumulated some mileage and is shifting poorly just to see if zinc alone would restore the original shifting smoothness but I have always just changed the oil rather than tinker with the old stuff.
Like most things regarding motor oil I suspect we will never really know specifically what is going on. The issue will continue to be debated and speculated on forever but there really is very little data out there to review. The people who really care about lubrication have collected a lot of data (the subject is tribology) but it seems to be based on the use of special and expensive lubricants. For example, NASA has a great deal of knowledge about what is required to lubricate things in the vacuum of space (a very harsh environment) but they also do not care to save money on lubricants. Same thing with motor oil, the people who really care about how it performs typically use good grades of known brand oil and maintain the engines carefully. What we really need is to see a few million vehicle- miles on different oils and to quantify the results.
Brian
This notchy shifting issue is a real puzzler for me as I don't see the scientific basis for it. Isn't one of the main features of Synthetic oil that it maintains its effectiveness longer than fossil oil? Can anyone suggest an explaination of what is going on to cause this notchiness?
