What oil is equivalent to Mobil SHC 636?

Mobil SHC 636 has just a couple of equivalents since it is a full synthetic gear lube with an ISO viscosity of 680 (about as thick as honey). They are Blue Sky Cirrus 680 and 76 SYNCON R&O Oil 680. However, both of those oils may not, and probably don't have the exact same additive package. The Mobil SHC 636 typically costs a little more than the other two alternatives but, if your warrantee is hinging on using the OEM specified oil, I wouldn't substitute it.

All of these oils are very high tech synthetics designed for small gear boxes (generally worm drives) under high loads and extreme temperature ranges. Non-synthetics are not a match and won't hold up. They are also tough to find in anything less than 5 gallon buckets, with the exception of the Mobil SHC 636 which is sold in small tubes on eBay and, I suppose, some other places.