I talked to Mr Zimmerman, of Zimmerman's Cletrac. He's the one that rebuilt my C4.
The issue he says is that the rear end has a bunch of bronze seals in it. He stated that the big deal is you need to have GL-1 spec oil.
I chose to go with Amsoil Powershift. It's a full synthetic that's made for steering transmissions like the Imp.
It's sure not cheap!
The achilles heel of the IMP is that rear end. It's the most expensive part. In talking to Mr Z, he said that the downfall is usually water in the rear becoming ice, and then snapping a tooth on the ring gear. One of the main culprits is snow going in the shifter boot on the top of the unit. Something to keep an eye on.
So.. whichever oil you use.. make sure to keep the water out of it! I'm thinking of pulling mine a couple times a year and letting it settle out ( or heck, even freeze the whole jug in the chest freezer) to make sure there's no water.
Or.. use something cheap, and keep throwing it at it!
Welcome to the forum. Read up a bunch on here. Lots of info.
JP