My E420 takes 8.5 quarts and so does my 380SE, so I would guess yours is also in the 8-8.5 quart range. I have not made the switch to synthetic and still use dino 10-40 or sometimes the 15-50 or 10-50( can't remember exactly) if I can find it. I also use the topsider to suck out the oil through the dipstick..works great and always get out darn near 8.5 quarts.

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95 E420 ( 76K) SOLD
85 380SE(125k)
95 SL500(72k)