I recently removed/installed the fan clutch on both my cars, an 84 and an 85 300TD (swapped for diagnostic purposes).

They are 10 mm bolts, that from what others have said, are quite prone to rounding off.

I used a the open end of a regular 10mm Craftsman combination wrench and about a 3 foot piece of pipe over it as a cheater bar, to apply strong, steady force. I had no problem loosening all 8 bolts.

As mentioned, rotate the engine so that each bolt is readily accessible from the top.

A good soaking dose of PB blaster wouldn't hurt either.

Good Luck