Hi all just wondering can the oil in the clutch migrate through the clutch rod tunnel, seem to have a bad leak on the left casing, doesn't seem to be the primary gear oil seal, although it is in that area maybe a seal /o ring missing somewhere along the line. 🤔🇬🇧

I think there should be a felt seal on the sprocket side. Thinner oils and higher temps will exaggerate the problem.

It could be leaking down the final drive sleeve gear. Early bikes had bronze bushes in the final drive gear to support the gearbox mainshaft, if they're worn oil from the gearbox could leak past. Later bikes had needle rollers and there is a small oil seal in the end of the final drive sleeve gear, again, if worn oil can get past

Oil from the clutch can get into the pushrod tube inside the mainshaft but it's unusual for it to leak out from there

The sleeve inside the final drive seal can also wear, the seal can wear grooves into it, so you could have leakage there. There is an O ring behind the sleeve but I'd be surprised if oil could leak between the mainshaft and bearing, even if the O ring was missing

Ninja -

+ 1 on woody's comments.

 If the leak persists, check that the gearbox breather is clear. It's a small hole high up on the left crankcase, just forward of the fixed end of the kickstart  spring. If this hole is blocked, some have found that this can cause an oil leak from the gearbox or worsen an existing one, especially when the motor gets hot.  (The air inside the gearbox will expand with heat, and it has to go somewhere, I suppose..........)

On this 199 engine I've pushed a piece of wire into the breather to make clear its position :-

HTH