Found this on Royal Purples FAQ:

My vehicle’s transmission specifies an API GL-4 gear oil. Can I use Max-Gear?

Yes. Max-Gear possesses both API GL-5 and GL-4 certification. Max-Gear is formulated with Royal Purple’s proprietary, Synslide additive technology to provide the exceptional film strength necessary for GL-5 applications, yet it is noncorrosive to soft metals found in manual transmissions that specify a GL-4 rated lubricant.

I also found this on the site:

Synthetic oils enable Royal Purple to make superior lubricants, but it is Royal Purple's advanced Synslide additive technology that gives Royal Puprle's EP lubricants their amazing performance advantages. Synslide additive technology truly is beyond synthetic.

Synslide additive technology, Royal Purple's tough, EP lubricating film, provides maximum protection under boundary lubrication conditions typically caused by heavily loaded, slow speed and / or shock load conditions. This tenacious, slippery film significantly improves lubrication and reduces wear by increasing the oil film thickness and toughness, which helps to prevent metal-to-metal contact in gears and bearings.

Synslide additive technology is noncorrosive to gears and bearings, including case-hardened gears that are easily pitted by conventional sulfur-phosphorus EP oils. Synslide additive technology displaces water from metal surfaces and excels in protecting equipment in wet environments. It also fortifies the oil against the detrimental effects of heat, which causes oil to oxidize.


*I have the Max Gear Oil*