I personally haven't had any problems switching to a synthetic oil on a used vehicle.
At about 50k miles, I began using synthetic oil in my VW Jetta TDI. It now is pushing 200k and still doesn't leak or use oil. Also switched oils in my Cadillac V8 at around 90k. It now has 110k with no adverse effects. The MB started using synthetic in 1994 and 100k later, has not experienced any problems oil-related.
One word of advice though, if you plan on running extended intervals, it wouldn't be a bad idea to use a diesel-rated synthetic. Not so much an issue if you change at 3-5k miles, but if you are going to use a 10k mile interval, the diesel synthetic will handle the soot load better. I've done oil analysis both ways and there really a clear difference.
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Sold: 1987 MB 300SDL, 320k miles, Astral Silver, Blue Leather, all books and records from day one.
Totalled: 1998 VW Jetta TDI, 488k miles, Classic Green/Gray Leather, loaded, E-Codes, "Slightly" modded, "Slightly" faster than stock, arrived 11/07/1997.
MBCA Member, Chicago Region