looks like I need to try some 5/30 as I have clutch drag problems! btw a lot of people seem to use ATF, though I havn't tried it

The 15w/40 should be OK for a ride or two. But the clutch may drag a bit from cold

Oil viscosity is measured by weight, hence the W in 40w so 75w oil is actually thicker at temperature than the 15w/40. A multigrade oil acts like a thin oil when cold and a thick oil when hot. I.E in this case 15 weight Cold / 40 weight Hot.

I used to use Putoline 75w gear oil but have switched to 5w/30, Beta recommend 10w/30 oil.

You will find that the gear change is a lot smother with that than 75w gear oil.

The W stands for winter: 10w40 this means the oils viscosity is 10 in the winter and 40 when it's hot

It's actually a lot weirder than most people think

Paul is right in the "Acts like" comment because they use chemical effects to make oil have different characteristics without actually having higher viscosity, it doesn't get thicker when hot

And gear oil being 75 or 90 wt actually have nothing really to do with viscosity it's merely to differentiate it from engine oils on the shelf

Anyway viscosity is just there because we consumers recognize it, it's less about pour ability than it is about other characteristics