after reading numerous recent threads on switching older cars to synthetic oil, i think i have things straight. well almost. the cars in question are both 83s. a 240d and a 300dt with 270k and 295k miles respectively. changed to mobil1 15w-50 over the weekend. drive 1000 miles and change the oil filter? then drive an additional 3 to 4 k miles and change oil and filter? thank you.