I would recommend against using a full synthetic oil in a high mileage engine to begin with.

Secondly, have you looked into the replacement of the tappets (lifters) ?

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1994 E500 (MY SUNDAY DRIVER)
1993 190E SPORTLINE LE W/ M104 SWAP
1997 C230
2002 ML320
2000 BMW 528I (WIFEY'S CAR)

"Excuses are crutches for the unfounded."