NewGuy, I wonder if they were looking at the wrong part. If the cover gasket is $72 I'd say that's way expensive.
Acewiza, I have a couple of questions:
Isn't the link you posted discussing a manual transmission (not a differential)? Is that transmission known to have overheating problems? The reason I ask is because I'm not familiar with that particular transmission. Is the factory fill spec lower than is optimal?
On my differential the spec allows a pretty wide range (5/8" to 1 5/8" below the fill hole) with 5/8" below the hole "full". I don't feel I need to go any higher than that and see no benefit to it. My differential does not have a known heat-related design problem (I'm not saying the g360 does but that was the impression I got from the links you provided).
If the factory spec is wrong or "not-optimal" then I believe we are talking apples-to-oranges (besides transmission vs. differential). I've not heard of any problems with the differential in my vehicle and I don't want to create any by overfilling it. I only took a couple physics courses between HS and College so I'm no expert but I did learn a little about fluid friction, energy and heat. The difference between a "properly filled" Tahoe differential and an overfilled differential in regards to heat generated may be 1 degree or 30, I don't know nor do I feel the value in testing that because I feel confident enough that a properly filled one will not have a fluid level related failure. If I knew that the factory spec was wrong or that my differential was prone to overheating, then I'd either start experimenting or I'd buy something else.
For my applications, yes, I believe that overfilling is a bad thing. For every application, well, maybe not, but your g360 may be the exception, not the rule. But we are talking about a Tahoe differential, the same diff as in my vehicle. To agree about disagreeing, I would just say, maybe it won't do harm but I see no reason to do it.
P.S. perhaps you were seeing lower temps because it will take longer to heat up the fluid since there is more of it. That does not mean that at some point the fluid temp would reach the same temp as previously experienced, or even go beyond that temp.