That seems a sound deduction Mr B.
After seeing the state of the oil that came out of the engine, and the fill, drain and level plugs on the bevel drive looking grubby, I drained the rear drive on the SP3. The oil was black, but then it has Molybdenum additives so it was always that colour anyway: I ran it through a paint strainer and there wasn't anything untoward caught in the gauze.
The drain & level plugs are plain steel so they got the plating works; the filler is chromed so wouldn't take the plate as well, but Gutsibits sell stainless versions fairly cheap.
Plugs came up well:
![](https://www.dropbox.com/s/t8ieh3d4sh7u734/IMG_20201010_140436.jpg?raw=1)
230ml of 80W-90 oil is now back in, and I'm waiting for the Moly additive to arrive to put 20ml of that in beside it, so it will be black as a black thing again.
I'm not intending changing the gearbox oil as that got pretty well cleaned out when I broke the clutch mounting rod & took it apart, so I know that's OK. Forks were stripped to powder coat the lowers. Brakes were bled through the other week when I rebuilt the rear master cylinder. I've now flushed all the fluids twice, or stripped & cleaned out everything since I got the bike, so I'm happier that there's no more manky surprises waiting for me. Oh: front right brake hasn't been re-bled - one for the next shift.