Just a few quick questions, from what I found 10w30 is what I should be using as gearbox oil in my 75 gt80.

Would synthetic be beneficial or harmful?

10w30 motor oil? or does it need to be a trans oil?

is 10w30 really the best? i know 10w30 used to be quite standard so I was thinking perhaps that something else could be better?

Thanks in advance for any help and for taking the time to read this

You just need to be sure the oil is ok for wet clutches, most modern engine oil has additives that make it too slippery for maximum fuel efficiency, and your clutch will slip.

I like synthetic, to me it feels like it helps make shifting smoother. With as often as the oil is supposed to be changed, (every 1k or so) I don't feel like it really makes as a big of a difference for the health of internal components.

The bike probably originally came with 30w, so 10w30 is a nice upgrade. I just get whatever motorcycle / wet clutch compatible oil the local parts store has, be it 10w30 or 10w40.

This topic comes up quite frequently on the aircooledrd forum (UK based).

I don't know what specs you have to have stamped on the product container in the US, but here in the UK, if it is stamped "MA2", then it is suitable for wet clutches.

sometimes it reads as JASO MA or JASO MA2

a cheap option is type F ATF