Don't know who you've been talking to, but I'm using Mobil1 synthetic, and I do not have any leaks from my 83 300D's engine.

Regular oil works fine in the trans and rear end.

Have you checked the tranny fluid level?

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles