Just completing a bit of a service on the old girl. I changed the gearbox oil and also the final drive oil. For the gearbox (approx. 850mL) I used a Nulon product, SAE 80-90, with the other numbers the same as per the Haynes manual. I didn't have enough left for the final drive, so I rummaged around the shed and found a Shell product, called "Spirax", also SAE 80-90, and the other numbers matching which I duly put into the final drive casing. My problem is that the viscosity is vastly different between the two oils. The Spirax is much "heavier", nearly honey consistency (a bit of an exaggeration!) while the other is more like normal engine oil, slightly heavier. Am I worrying un-necessarily? Will the final drive casing explode? I haven't ridden it yet, pending comments on this wonderful forum.

Thanx, Rick G.

Just be sure they're both rated GL-5 Hypoid for the helically-cut gear faces. I use good quality synthetic 75W90 GL-5 in both units. It doesn't matter whether it's crushed, squeezed dinosaur juice or synthetic in the end, as it's merely personal choice. I like the way I perceive that the shifting is made slightly easier with synth in the gearbox.

Hi Rick,
The GL-5 is really critical, please make absolutely certain the oil in there is 100% GL-5 rated if it isn't you are in for an expensive lesson down the road. The amount used for the two places is so small buy premium GL-5 rated gear oil and replace both I use a Castrol full synthetic and I know others use Mobil full synth, all get excellent results.
Regards,
K-BIKE

Okey Dokey...will do! I have drained the Spirax stuff from the final drive unit. Will drain the gearbox, even though it is labelled GL-5 Hypoid. Will go to the Castrol stuff.

Thanx!

Hi folks, is there a recommended replacement interval for the gearbox oil? I've no idea how old mine is, but I have got a few spare litres of 75W-90 GL-5 semi-synthetic, so I'm tempted to change it.

Anyone know if semi-synthetic is compatible with older mineral oil?

Cheers,
Steve

I drain and refill my gearbox and final drive oil once a year whether it needs it or not. It is hydroscopic (absorbs water) and I tend to ride 20,000 to 30,000 kilometres per year at a minimum. It's cheap insurance. I don't mix brands or types but if you drain it warm it'll all drain out sufficiently. A 'few spare litres' will more than do both ends, no worries.

If you are really worried about the tiny amount of old oil mixing with the fresh stuff, change the oil in the gearbox and final drive, ride for half an hour and change the oils again.

Then keep on using the same oil.

Im going to change my oil in the gearbox etc soon too, is this what you would recommend, Castrol Differential EPX 80/90

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_155319_langId_-1_categoryId_165750#dtab

Thanks

the spirax is known well in the "big machines".... tractors and trucks rick ..
i remember seeing it in the sheds of the "best machine shops" of my relatives farmer friends

castrol is a bit ho hum for me ... my father used to be a sales person for castrol years ago (country nsw rep )....but it isnt the same product any more ...but the ...gl5 rating of castrol epx80/90 should do the job

The crunch for me in this is the opinion of Tom Cutter in the US who rebuilds more gearboxes than anyone else that I know of and he has consistently recommended the Castrol product. I used to use Redline and after some discussion with Tom I went over to the Castrol product.
Regards,
K-BIKE

We did a fair few gearbox and final drive rebuilds after the city and state cops started using Redline's gear oils in their own maintenance shed on the R1100RT and R1150RTs. This was in Portland Oregon and a few years ago now. It was not a pretty sight inside of some of them after draining that goop out along with slivers of metal. Castrol's EXP 80W90 (hypoid GL5) comes highly recommended.

We used Castrol products on our military vehicles, obviously designated under military specs and classification but we had no problems at all.

BIG D