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) ?
1994 E500 (MY SUNDAY DRIVER)
1993 190E SPORTLINE LE W/ M104 SWAP
2000 BMW 528I (WIFEY'S CAR)
"Excuses are crutches for the unfounded."