Good evening. Today I checked the bevel drive/ shaft drive/ final drive (?) on my new ( to me ) 1979 T3 California. There is a leak from the inspection plug. I took it off and black oil is nasty looking; so I decided to give it an oil change. Whilst I have a manual that states adding a portion of Moly to the 90w it is an old manual. I presume (?) that modern oils will suffice without this addition ? Looking here I cannot locate any info ; I have tried without success by putting in search terms such as Viscosity/Shaft/Bevel etc. oils etc. etc. however I have found no thread whatsoever in regard to the preferred or acknowledged oil / viscosity. Searching the web I found reference to 85 /140w ? --90w --85/90w-- who knows ? I certainly do not. Can anyone please guide me here to a simple answer ?
I still use 85W90 in my drive boxes (you can’t get SAE90 anymore as far as I know) plus Moly additive from Halfords. It’s really not a great additional expense. I’ve heard people say that the additive’s not needed with modern oils. My question would be, which modern oils? I’ve only left the additive out once when I couldn’t find any. It might be just a coincidence but that was when the pinion gear broke up and lost some teeth.
Same here. Only thing I’ve ever looked for in Halfords, fully expecting not to find, but did.
Got a little ‘classic oils’ place near me that sells 90W gearbox oil too, so I’m still doing what I always did.
Thanks for the replies.
I have searched further and found Castrol still makes and sells 90w. A company called “Lubetech” ( https://www.lubetechshop.co.uk/ ) sell Exol oils . Colin is a very helpful guy, I’ve bought different oils from him for a long time; they also sell straight 90w. The best find, however, is Wilco. ( Not Wilko btw ) They sell Comma oils, and list two 80/90w gear oils. I contacted them, ( Comma ) they suggested the EP80/90 GL5. It’s expensive @ £4.99 
1 litre.
The big blocks use “90” oil in the bevel box - I got my last lot from B&Q who were flogging off some old stock Castrol stuff for £1/litre so I cleared the shelf for them. Only 1 left now sadly.
I vaguely remember a cautionary note about GL5 compatibility with yellow metals, but would have to go hunting for anything more useful.
The small blocks use “140” oil - I think they run hotter. I have only come across multigrade versions of that, e.g. 85W-140.
Ian
An oil thread! 
Just for the record, my Spada had Castrol Grand Prix, because it was closest match (according to Castrol) to the originally specified SINT 2000
G/box & FD had SAE 90 (again Castrol?)
The later 850 LM went with Silkolene 4-stroke oil 10-W40 & as above for transmission.
Then I switched to a 20-W50 because of it getting very thin in hot weather and the oil pressure light getting very friendly & winking at me a lot (but only at idle).
But latterly the g/box & FD both got a SAE 85-W140 (because was recommended on Guzzi club forum a few years ago) plus a dash of Molyslip in each (10cc FD 20cc g/box IIRC); the g/box had slicker & smoother gear changes as a result, the FD seemed to run cooler, whereas before I couldn’t leave my fingers on it after a good run.
HTH
https://www.wilcodirect.co.uk/product/gear-oil/comma-ep85w-140-gear-oil-1-litre-co-hmg1l/CO-HMG1L ( The plot (?) thickens )
It seems GL5 was initially not suitable : however modern oil production has it sorted. see 1.40 here…
[
But latterly the g/box & FD both got a SAE 85-W140 (because was recommended on Guzzi club forum a few years ago) plus a dash of Molyslip in each (10cc FD 20cc g/box IIRC); the g/box had slicker & smoother gear changes as a result, the FD seemed to run cooler, whereas before I couldn’t leave my fingers on it after a good run.
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I found this to be the case too with my 1000S.
Lack of information on the forum is probably due to loss of a significant amount of archive material a while ago.
Switched to 85/140 with Moly added for all Guzzis some years ago.
Based on not wanting to stock two lots of oil and long conversations with the tech people at the wholesaler/refinery/blender that I was buying my oil from.
No problems to date with wear or leaks.
Steve