RRS-1, the bottom cover gasket is paper and the bolts are often over-torqued so the gasket gets crushed into pieces. Which then leak. Other possibilities are loose bottom cover bolts or end-cap studs; the problem with these is trying to get the area clean enough for sealants to adhere, unless you drain the lube and carefully solvent-wipe everything. Just slobbering sealant on a place where 90-wt is leaking will usually do nothing.
Some owners have had good luck with sealing-washers both of soft copper and special steel washers that have bonded neoprene rubber in their IDs. Pegasus-Racing in WI sells the bonded washers and motorcycle shops stock dead-soft copper washers. McMasters-Carr in CA may also have both.
Finally, ZF specifies regular 80W90 but some people use synthetic lubes for supposed shifting improvement, which sometimes leak where non-synthetics do not. There are several different kinds of synthetics- some swell seals & gaskets, some shrink them and a couple do neither. The shrinkers will leak; unfortunately, manufacturers will not tell you which kind of synthetic they make.
