I'm not sure if there's an equivalent to 80w or 90w automotive lubes. I'd imagine an all-steel RC diff could use them in a pinch if all you wanted was a near-locker. I can tell you that the bigger the number the thicker it is. 1000w is so thin that you might as well have an open, it would only be there to lubricate. 100,000w fluid would be so thick that the diff would behave as if it were a locker.

As for the grease...not sure TBH. Could be, I've seen people use it to seal unsealed RC diffs on here before.

As for the shocks, it's pretty simple. Drop the car from about three feet. If it bounces, they're not filled. If it doesn't, then they are.