Gearbox oil

I've noticed the gearbox bung at the top was missing from my mk1 and was wondering if there was any way of checking the levels. Before I completely drain it and add new oil. Thanks 

That bung on top of the gearbox is not for gearbox oil, it's for timing the engine and you can see the fly wheel, do not add oil the this hole or you will coat the clutch in oil….

Info on what is what on an 020 gearbox.

020 Oil

Gearbox oil change is A good idea, 40,000 ish miles between oil changes is worth it and change to fully synthetic which help gear change when cold.

Stuff I've been using.. get it on discount for better price….
https://www.eurocarparts.com/ecp/p/car-accessories/engine-oils-and-car-fluids/car-tranmission-oils/manual-transmission-fluids/?522776022&0&cc5_868

But make sure you can undo the filler on the side of the gearbox before draining the box from the drain at the bottom! If you can't it is possible to refill the box from the speedo cable drive port at the top.