You don't need an expensive "synthetic" 75W-90 GL-5. Any 80W-90 GL-5 is just fine. Brand makes no difference. Just make sure the label includes the API "donut" with SAE 80W-90 GL-5 spelled out. DO NOT USE ANY GL-4. GL-4 lacks the EP additive (what gives GL-5 its pungent odor) required for hypoid gears.

If it's a 16V and you're doing a complete axle fluid change you'll need some limited slip friction modifier - same as what any cars with clutch type LSDs use. The NAPA p/n for a four ounce bottle is NCB4285V.

Duke