2003 Hyundai Santa Fe Differential oil?

I was leaking very, very thick (like molasses) fluid from the front center of the engine. Mechanics say it's a differential seal. Super expensive to fix. We've been refilling it with 80W90 gear oil but this stuff is way thinner than the stuff that was leaking out. Is this the right fluid to put in? If it is, what was leaking out before (the car is no longer leaking the thick oil, they drained the differential 2 times back and refilled it completely with just the 80w90.)

Hyundai Santa Fe 2003 AWD 2.7L

Thanks,
Matthew