[KTL]

3-3/4 quarts is a lot of oil to add. You're probably overfilled since the factory documentation lists "approximately" 3 liters of 75W90 API GL5 gear oil as the capacity.

1 liter = 1.056 quarts and therefore 3L = 3.168 qt. You're a little over a 1/2 quart overfilled.

I would think any synthetic is going to do well for you since the factory fill was non synthetic. Everybody has their preference and the bigtime racers who prefer Mobil 1 say the best formula of that brand to use is Mobil 1 SHC 75W90. I tried it and noticed no major difference this year at track events (DE).