I have searched for the same question.

Spec calls for '90 weight Hypoid gear oil'. You can find dino versions of this at Autozone etc.

Most use 75W-90 Mobil 1 Synthetic Hypoid Gear Oil. Also at AutoZone etc. After reading all of the info on oil here, this is what I use. Its 5 bucks more to use the full synthetic.

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car