The only theoretical concern about switching to synthetic oil is that the additional "detergent" agents may remove sludge that is plugging up worn-out gaskets.

I switched my GTi over to synthetic when I bought her with 120K on the clock. The oil pan gasket leaked a teensy bit on the first cycle of synthetic oil, but the problem resolved itself within 2500 miles.

Given my experiences, I advise switching to synthetic oil and replacing any seals that are truly worn out. Just follow the owner's manual and buy the correct grade of motor oil.

Also, change your oil regularly and use a quality-built filter.
FRAM filters are crap; I use Wix or Hastings or Purolator.