The top gearbox manufacturer ought to know, as you say. It baffles me that modern gearboxes obviously have synchromesh and don't suffer this problem; do they use a different material
F123C. ?
Hi Peter, You woke me up by mentioning my name - I thought I was in trouble!
I haven't had cause to open up many modern gearboxes but it appears that at least some are using the Porsche type synchromesh (as used on Fiat 127, 128 etc). This doesn't use a brass synchro ring, it uses what seems to be a sintered steel? split ring without teeth (there's very slight circumferential grooves inside this ring which 'cut' through the oil film) which has to compressed by a sliding hub before a gear can be engaged. Also the gear teeth tend to be smaller, more in number and supposedly better finished. Such gearboxes use a light oil, sometimes SAE 10 viscosity, sometimes auto-transmission fluid (similar in viscosity to SAE 10), many now use a synthetic oil designed for long life.
Afaik, there's isn't a lot of difference in viscosity (a measure of thickness) between SAE 90 gear oil and SAE 50 (or SAE 20w50) engine oil when it's heated to operating temperature. SAE 20w50 engine oil is an oil that is as thin as an SAE 20 oil at low temperatures (so easy to turn over the engine and the oil flows quicker to the bearings) but as thick as an SAE 50 oil would be when heated to operating temperature (c. 90 C. ?) so provides good lubrication when hot. Bear in mind that an SAE 50 oil will thin out as it heats up, so will an SAE 90 gear oil....
The sales blurb, back in the day, when multigrade oils (e.g. 20w50) were introduced said that the above ability was due to the incorporation of 'long chain polymers' (whatever they are :yum
. As regards using such oils in fwd cars e.g. the Mini, it was said they provided good protection to both engine and gearbox but the oil change interval should be adhered to, as the action of the gear teeth 'chopped up' the long chain polymers.
Re:- lack of easy availability of suitable non EP gear oil in Australia? I'd suggest OP might try a truck workshop to see what they use? Or try a motor factors that supply the heavy commercial workshops.
The only concern I'd have about using 20w50 engine oil in a Fiat 500 gearbox is that there might be a lightly higher risk of developing an oil leak e.g. from the driveshaft oil seals, plus it might make the gearbox slightly noisier? I've used 20w50 engine oil in Fiat 124 gearboxes (rwd) although iirc SAE 90 (non EP) oil is recommended. The result was improved gearchange quality but noisier, especially at tickover.
Molyslip used to do an additive for gearboxes which was supposed to quieten it. (they had a different additive for axles that used EP oil). Maybe there's an additive still available that might help protect the crownwheel and pinion gears?
Someone mentioned that EP oils shouldn't be used in car gearboxes because they damage 'yellow metals'. I seem to remember, EP oils can damage Bronze bearings etc. I don't recall Brass being mentioned (afaik many synchromesh rings are made from brass?). But I do remember reading that the reason for not using EP gear oil was that the synchromesh ring grooves would have great difficulty in cutting through the EP oil film, therefore the synchromesh would cease to work.
AL.
