Here’s what one website says. Maybe that application results in an unusual amount of varnish-forming byproducts.
"Most gear oils use a polyalphaolefin (PAO) as a base stock. This is essentially a man-made version of the naturally occurring mineral oil pulled from the earth and refined. The size of the molecules are all relatively the same. These are also saturated molecules, which makes them very stable.
The benefits of synthetic oils relate to the molecules’ stable nature and include a higher degree of hydrolytic stability and demulsibility, a higher viscosity index (VI) and a lower pour point. The VI and pour point allow these oils to perform better across a broader temperature range.
The drawbacks of synthetic oils would include their cost and solubility concerns. PAOs generally have low solubility, which means they do not dissolve additives as readily as their mineral counterparts. Synthetic oils also have a difficult time suspending varnish-forming degradation byproducts."