Hi Shaun,
Welcome to the Forum,
Any non detergent or low detergent oil will be fine, opt for one of the "classic" types either 20/50 or straight 40
I feel its a right pain kicking and kicking to get the oil circulating, doesnt do the mechanism any good either
remove the plugs and short the leads out to the engine to avoid stressing the magneto
I fill with oil the night before If I can to give time for it to make its way down to the pump etc
then push it in gear up and down the road til the oil returns,
(a set of starting rollers is next on my list
)
Also check the gearbox oil and primary case levels, a lot of owners use ATF in the primary, I use motorcycle oil for 4 stroke engines 10/40 as this is designed for wet clutches
SAE 40 or ordinary gear oil in the gearbox
Good Luck with it !!
John