All good. Thanks for understanding.
I've spoken to Eaton twice about this. The first time, they told me it was due to the lower friction coefficient of synthetic oil causing issues with the helical gear LSDs. The second time I was told "synthetic is too thin" which is just ridiculous and made me not want to buy from them again.
The first one I disagree with since the friction coefficient between group II conventional and group III and IV synthetic is not that much different. In a sliding couple with borided surface vs rotation speed, the difference between conventional and synthetic is ~0.010 (0.165 vs 0.155) after ~8 minutes at 20 Mpa load.
See white paper titled "
Experimental Analysis of Tribological Processes in Friction Pairs with Laser Borided Elements Lubricated with Engine Oils" published in December 2020.
https://www.mdpi.com/1996-1944/13/24/5810/pdf
The second reason they gave is just asinine. It takes 30 seconds to google and disprove that one. A 90 grade gear oil is a 90 grade gear oil viscosity regardless of the base oil.
Where the issues with the helical gear LSDs would show in elastohydrodynamic lubrication down to boundary lubrication which is where the additives play much more of a role than the base oil. What they likely don't want people doing is buying something like Mobil 1 LS 75W-90, which contains a friction modifier additive package, that will alter the dynamic friction of the gears under load. They don't want people using the FM, not the base oil. There are synthetic base oils that don't contain a FM pack. Red Line 75W-90 NS and Amsoil Severe Gear 75W-90 are good examples of synthetic gear oils that do not contain (or contain very little) that FM pack.