The reason the mfgr used the thick bentonite grease is because it does not need seals in the trans and it is waterproof. Many lithium based greases are not waterproof and will break down with long exposure to moisture. Not uncommon to open a 20 year old trans and find some water in it with the grease and the water has not affected the grease.
Is there any way to determine whether the grease in the new transaxle is actually bentonite grease? As far as water getting into the transaxle, I don't really think that would be a problem for me. I NEVER let my mower (or any of my lawn care equipment) get wet, don't let it sit out in the rain or anything like that, it's always either parked inside or covered with a waterproof cover. And I never mow wet grass. So, for me, the only potential problem would be not water getting
into the transaxle, but the 00 grease getting
out of it. And, as I mentioned, from what I have read it doesn't seem like that would actually be a problem.
But just to cover all of my bases, I've been considering using a liquid gasket material. Haven't really decided on which one would be best for this yet, still doing research, but something along the lines of Permatex The Right Stuff Gasket Maker. Would put a very thin coat on surfaces that press together to 'seal' the transaxle. The potential problem with this is the thickness of the material affecting the fit of the bushings and whatnot. Although when the upper and lower housings are tightened together the difference should be minimal. This is just something I'm considering, it may fall to the wayside.
I've also been reading about John Deere Green Corn Head Grease. It's 0 so it's a little thicker than the 00 stuff, but still thinner than regular grease. I've read some good stuff about it. Found a good source of information about all the various greases available in this thread
https://www.lawnmowerforum.com/threads/the-daily-yardman-thread.39996/ Its about someone rebuilding an MTD transaxle. It's a
really long thread and they get completely off-subject a LOT (and I do mean a
LOT), but it does have some good info.
What's really frustrating for me is that I wouldn't even need a riding mower if my tired old body could still push a mower without falling apart. My hip and back just won't let me do much of that anymore (plus I'm currently recovering from four broken ribs, a punctured spleen and a concussion). Growing old gracefully is a myth!
But the issue of grease is definitely taking a back seat right now until/unless I figure out the shifting problems. And I'm beginning to lose hope on that. If this drought continues I might just set the lawn on fire.