oils

Using the search facility you'll find a number of threads on oils. Every motorcycle forum I know of has at least half a dozen oil threads. Usually known as opening a can of worms. Self isolation due to the current pandemic might allow you to wade through this enormous amount of opinions, guesses and rarely even some proven facts.

My thoughts about gearbox oils. The Norton recommendation of a straight mineral engine oil (viscosity depending on ambient temperature) was the best then. When multigrade mineral gearbox oils appeared, they became a good choice. Note that a SAE75 or SAE90 gearbox oil has about the same viscosity as a SAE40 or SAE50 engine oil. When coming to additives in the oil, things are going more difficult. On modern synthetic oils, a number of people has the opinion that GL-4 is better because GL-5 oils can be nasty to the yellow metal bushings. Personally I use Motul gear 300 Oil 75W90 GL-5. It is recommended by TTI for their gearboxes. According to the data sheet it don't harm bronze bushings.

For the engine, it all depends on the type of rider you are. If you like relatively high speed and high annual milage, go for a 15W50 oil such as Mobil 1 Racing 4T. If not any good quality mineral 20W50 will do. Opie oils website has a good range of products and good specification information.

At the risk of upsetting traditionalists, I would not use 'Classic' monogrades

It very much depends on the state of the engine. If it is a fresh rebuilt engine where all internal oil passages are cleaned, including the flywheel sludge trap and the oil tank, you can get one answer. If you have added an Commando type external oil filter, another oil might be the best choice. If the engine has been used with a classic mineral, the change to a modern oil could loosen up a lot of residues. They can block oil passages or accelerate engine wear. So another best oil recommendation. If your engine is prone to wetsumping, a multigrade will wetsump more than a straight oil.

Remember that the oil system was designed for frequent oil changes.

In my 99  over the last 25 yeas I have used  a diesel  HD 30,  classic 40 ,classic 20/50, and synthetic 10/40.  The motor has not been apart and goes well. The box has had HD30,classic 40 ,EP90 and  ATF.  Nothing has worn out  as yet. I'm surprised that the 10/40 (Yamalube)  seems well suited but may change to XL 20/50 if  some long distance summer trip comes up.I change oil before it goes black.and have several magnets in the system.Nothing has ever turned up on them I plump for XL20/50  and EP 75/ 80 in the box..Basic 10/40 or 20 in the primary.  Farm shops stock 20 monograde.