Charon wrote on 11/04/09 at 14:17:04:There are actually quite a few motorcycles with separate engine and transmission lubrication systems. Old British bikes had separate units, and I believe the current Harleys are also separate.
Way back around 1960, if the stories are true, the British Mini was designed with a common oil system for the engine and transmission. The only required change to the transmission was replacing bronze synchronizer parts with steel ones. Few motorcycles have synchronizers in their transmissions.
In the middle '70s, in response to one or another fuel crisis, the Army changed out the heavier oil in truck transmissions to use engine oil. It's been a long time, but I think they changed from SAE 50 in the transmissions to SAE 30. The change was said to have no deleterious effect, and to improve fuel mileage by a small amount.
Originally, all motorcraft, be they 2, 3, 4 wheelers, had gearboxes separate from the engine.
So engine oil was separate from gearbox oil.
The story about the Austin Mini is correct.
Whereas the Austin Minor had a 4 cylinder engine mated to a traditional gearbox and rear wheel drive (so engine and g/box would be separate)
the extremely tight space available within the Mini's engine bay was such that the solution was to fit the engine transversely, and design a special gearbox that was integral to the oil pan (sump in English)
Thus, the first truly front wheel drive, transverse engine system was born.
Competitor marques (such as Renault) insisted on a longitudinal engine and a transverse gearbox... definitely a more primitive design.
Motorcycle gearboxes, unlike automobile gearboxes, have sequential gear changing; you must go through all the gears whether going up or down.
Cars & Trucks have the typical "H" layout which allows to shift 5th-3rd, or 4th-2nd... not possible with a sequential system.
Most motorcycles today have a wet clutch, so "engine oil = gearbox oil" is the paradigm.
Ducatis, Guzzis, BMWs, even Harleys have a dry clutch so you will have separate engine and gearbox oils.
I have never considered a 30w oil in a gearbox; both my truck's instruction booklet, and that in my wife's car insist on 80/90 oil.
It is unlikely that I may come across any 30w oil, but a cheap 15/40 could be an interestnig idea...