The problem with using higher viscocity gear oils in these gear boxes is that the oil pumps are likely to fail in cold conditions - not a serious problem for much of Australia, but sooner or later it may happen. If it does there is no indication - until other bits fail under load maybe tens of thousands of kilometres later.
The load on gear teeth is low compared to differentials for example, and almost any lubricant is adequate from this point of view, same for bearings. At high loads, particularly in hot conditions the cooling properties of the lubricant become important, hence the oil pump, but the difference in this between oils is very little - just a matter of circulating the oil around.
Where the oil becomes more critical is in the performance of the synchromesh and gear shifting in general. This depends on the slipperiness and adhesion properties of the oil, and quite small changes can greatly affect the feel of the gearbox, hence the variety of recommendations as the manufacturers try to get the best all round performance. But we are not talking about failure of gears or bearings. Most gear and shaft failures, and probably some bearing failures result from shock loading of the drive train due to extreme conditions, lousy driving, or both. (excluding design problems that prevent adequate lubrication!)
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
Bookmarks