CENPECO - AKA Central Petroleum Co, Cleavland OH / Walcott IA. Big into what they say is heavy duty applications. Strong suit is high parrifinn based mineral oils. They take common Lubrizol Corp and Ethyl Corp SAE / API additive packages and blend then into Shell base oils. The GL4 is a synthetic blend and GL5 is a straight synthetic (normally a PAO). The Mobil SHC is a solven hydrocracked oil - basically a mineral base oil that is stripped of the waxes and the sulfur to the point were it is tough to tell it apart from true synthetic without some sophisticated mass spectroscopy to determined the molecular weight, distribution and chain structure. Keep in mind, Mobil, Shell, Texaco, Citgo, BP, etc.. could all make some fantastic oils. They don't. They can't. The more you put into the oil, the more expensive it gets. These guys sell at point of purchase (Autozones) and through distributors. It is all a price point play at that level.

The OEM spec a weight (viscosity) for a reason. I am not saying they are right or wrong. To determine the correct viscosity for a gear oil, you have to know servel thing, the torque, the speed, the size of the gears (drivers/drivens), the operating temps and the low temp start up. I would start by using waht they spec in but that is n't the place to stop. If you use a 85W140 in a box that calls for a 80W90 or 90W then you run the risk of haveing an oil in there that is too thick. Adequate thin streangth may not develop and you can have bigger problems, conversely if yuou go wit a 80W90 and it calls for an 85W140 then the oil is way too thin and over heating will occur for starters but it won't end there.

If it was in anything smaller than a 5 gallon pail in north america, it was regular M1 gear oil, not SHC.

Also - thinking about it, I may have spoke in haste. GL4 although many are blends are really defined by certain specification levels and the GL5 actually are a different level of specification levels. It really has little to do with the base oil and more to do with the additive package used. It just so happens that many GL5's are synthetic and many GL4s are blends but it should not be considered the definition per se.

Going a bit deeper. The GL-4 oils have only half the quantity of additives and EP agents that are in a GL-5. The sulfur and phosphorous contents are high enough that the use of a GL4 where a GL5 is specified can have adverse effects on the gears. This is particularly true of gear elements containing yellow metals (syncros?). (Check out my thread on copper components in the engine rebuild section). The GL-5 designation was really designed for the automotive industry and the GL4 was for non-automotive or off road applications.

Yes, Borg Warner type synchros typically contain yellow metals. but 915s don't, so we don't have to worry about sulphur/phosphorous additives.