Since we're on the subject of P80, I'll take the opportunity to repeat my testimonial regarding the use of this stuff on fork gaiters. It's been a few years now since I've been using it, and the gaiters are holding up very well.
As to my original query, I've ordered barbed brass "reducing" fittings, 3/8" to 5/16". They're coming from China, so I'll be waiting a while for them, but that's ok; I'm not planning to install the oil filter until the riding season is over. For appearance's sake, I'll first try them without clamps. (There WILL be clamps on the oil filter spigots though.)
Thanks NickL and Jon W. for the tips on where to find these fittings.
'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.