Had an oil weep at the 30mm crankcase inspection cap. Removed the cap and found a big, fat O ring with a squashed edge to it. Found a genuine Honda part on ebay for £2.50 When I compared the two, the difference was substantial. The genuine part is 29.7mm x 2.4mm The one fitted to the bike was 26mm x 3.5mm
I know Honda parts can be verging on extortion. But penny pinching for the sake of an O ring is madness.