Hi Steve,
Heres my 2 cents worth
Fork oil was developed because there was a need for it!!,
I use medium (10W) and adjust the amount to suit myself, I'm using damper rods as well in the forks
Add or drain some till you are happy
I would stay away from synthethic or semi for the engine until it has several thousand miles on it
In actual fact there is no need for synthethic oils in our BSA's, These oils are developed for long service intervals in modern high performance engines
I think everyone here changes their bikes oil very frequently probably at 1000 miles or at least once a year if less milage
I run my Super Rocket on straight 40, no issues over 10 years, and its ridden fairly hard
I use gear oil in the gearbox
and 10/40 MOTORCYCLE oil in the primary DO NOT use car oil as it will lead to clutch slip
Others prefer ATF and its fine as well
HTH
John O R