Adding Gear/Transmission oil to a Kubota L2202?

The 3 point linkage raising & lowering on my Kubota L2202 tractor is not working very well, it’s hard to shift, and does not seem to raise to full height.

So I checked the transmission oil by opening the oil plug, and there’s some oil in it, but only about 1/3 full.

There doesn’t seem to be any sort of dipstick for the transmission oil level. How do I know how much oil to add (if any), and what sort of oil should I add.