I doubt that that is the kind of thing that any manufacturer has placed a "number" on.
It would depend a LOOT on the lube you were running and the gears, etc.
They will take a lot of abuse. I think the general rule of thumb is that you should be able to lay your hand on the cover. If you rip it back off immediately, it's too hot.
I run synth 75-140 and even in California, can pass the above test (except during the break in period on the gears).
bburge