However, My latest understanding is that the 4 litres in the tank under the seat does both the engine crankcase and the gearbox. The 1 litre that goes into the primary casing is purely for the primary chain and the wet clutch.

Anyone want to confirm or correct me?

The gearbox and primary are one unit and use the same oil (in my case Redline Transmission Oil)

I got a service manual on line for about $8 and its the best thing.

Worst case you could always purchase the service manual for your year and model but thats taking the wooses way out!

My advise to the OP is to get the proper manual for your model and year of bike and follow that, because if you follow some of the advise given here you will fuck your transmission in no time at all.

Gotta know when to walk away from a thread and this is one of them.  

The service manual gives instructions for changing the oils or checking the level. However, I did not see anywhere that it tells you which oil covers the gearbox. I can't see that it matters apart from just someone like me who thinks about stuff. So long as you check both the oils and replace them correctly, you won't harm the bike simply because you have an incorrect understanding of which lubricant covers the gears.

Update:- Got the filter off with one of these.................

Had it in the toolbox for 30 years. Allows you to come at it from the end by using an extension piece on your socket handle.