New oil filter and rocker cover gaskets arrived today, so they got fitted and I refreshed the oil again.
All seems to be staying inside so far, though my poor neighbours have had to put up with a shouty flat twin once more. Aircooled cars are LOUD!
Oil recommendation is straight 30 grade, though 20 in the summer. I was warned away from synthetic as it can apparently cause problems with the oil seals. However, I ran out of 30, so she's got a mix of that and semi-synth 10w40. Started absolutely instantly (after an ignition-off crank to build up oil pressure), but she's definitely chugging a bit on tickover. I left her idling for a bit, and the nearside rocker cover was much warmer than the offside, so that iffy valve theory may be true after all. Open the throttle and she chugs briefly before clearly firing on two. Can't say I much fancy pulling a head off, so wondering if some Redex abuse may help.
Also received new (well, used) lower plates for the engine. They're not great, so I really am going to have to strip those and paint them I suspect.