I do not doubt the success stories I've read here and elsewhere regarding synthetic oil. I ALSO believe the "motor now leaking like hell" stories.
I have 2 VERY high mileage Japanese cars that have done well on "classic" Castrol for lack of a better word. I have 2 LOW mileage cars, one MB; the other Isuzu that went down the tubes when I swicthed to synthetic. Oil consumption went way up in the latter case.
My 2 cents.