What make & grade of oil is best for my Commando gearbox?
1973 750cc Commando Interstate
like many others I,ve tried many fancy synthetic oils in my Mk3 gearbox in the belief the latest technology must be better. Some of these oils didn,t feel quite right or improved the action despite their claims. I decided to try good old Castrol EP90 and the gearbox has never run sweeter. Selection is smooth with neutral easy to find. The oil stays clean and will easily last the recommended 6000 mile interval in the manual without problems. I use it all year round and no trouble with e starts whether hot or cold.
Synthetic 75w90 seems to reduce gear and bush wear, e.g. Millers Oils TRX Synth 75w-90 Fully synthetic gear oil
https://www.opieoils.co.uk/p-60227-millers-oils-trx-synth-75w-90-fully-synthetic-gear-oil-gl-4-gl-5.aspx - other makes and suppliers are available.
Most important thing seems to be to change the oil regularly, especially if there's any sign of water (usually via the clutch cable and breather hole).
I would go for the thinnest EP on the grounds that the plain bushes need the oil to flow well.
Actually, I think our Nortons rarely run so many miles that choice of oil matters. Though some oils give smoother gearchanges.
I believe it was Mick Hemmings who attributed rust on the gearchange plate, springs and screws to a clogged breather hole.
Some early GL-5 synthetic oils where not friendly to yellow metal bushings.
Mileage and riding styles definitely variable but good lubricants still important for rust protection as well. I,ve run a short silicone tube from the gearbox breather stub and the clutch cable is seni sealed so water can,t enter and has never been blocked, but there,s always condensation. This also applies to the primary. EP90 has shown no signs of water or wear and my bike gets ridden regularly, over 14k miles since the head rebuild in June 2018 even though it,s not my main vehicle
Mileage much lower this year after the much missed Evora Rally and my Flanders Poppy Run cancellation plus working from home since March. Still managing 1-200miles weekly. Off to sunny South Devon this evening for a nice mini break which will round off a pleasant 400 mile week
Have to agree with the Mickey Hemmings comment, my gearbox breather hole was blocked by solvoautosol and every metal part above the oil line including nuts inside the first cover were solid rust. The parts behind the inner cover were fine. Having checked and replaced the rusty parts it runs very well on EP90.