The manual is correct, 10-40 is a good choice-DUH!
check the cars records and use that as a guide to oil preference.i'm with BUD -if you're going to synthetic(especially if prev. owner hadn't used it!) make sure you go with a heavy one-stay away from 5wxx or 10wxx.when i got my mb i also wanted the best and i changed to 5or10 weight synthetic,but because my p.o. hadn't i ended up leaking at the top end-not much but i hadn't leaked at all before or since.i'm sure someone can give you the technical explanations but i figured the seals and rubbered bits dried out and shrunk and this let the thinner flowing oil past.
'95 s500 (bought for wife but can't bear to share!!!) 125kms
'92 legend 180kms
'88 tbirdturbo(fantastic car-only regular maint.)120kms
'87 mustang gt(gone)
'85 mustang gt(gone-but not forgotten)