I am not an oil expert. I have read many posts on this site concerning oil. The following is a short compilation of my readings. The 1.8T is very susceptible to oil sludge build up, especially the 98-99’s. Posts on this site have stated that synthetic oils contain more/stronger detergents. Your engine may be experiencing sludge build-up. The synthetic’s detergents may have knocked loose some sludge particles, which are now the genesis of the noise.
There are many people on this site that swear by a product called Auto Rx; it is an oil detergent that is suppose to clear out oil sludge slowly (preventing the clogging of your oil pump and screen => engine death). The makers, and users, claim that after using auto Rx engines have run cooler (since the sludge is gone) and quieter. I would recommend running a search on this site and read our member’s remarks. One of the posts actually has the link to auto_rx.
I switched to Mobil 1 synthetic at about 24K. I have run Mobil 1 5W30, 10W30 and 15-40W, all without abnormal noise or heat. I now have ~65,000 miles on my B5. During my next oil change, I will be using a WIX brand oil filter, which adds about .5 qt oil capacity to our pitiful overall capacity, and possibly auto-rx (I have a ’99).
Here is another oil expert page:
http://bobtheoilguy.server101.com/index.html … may be worth your time. I believe that they host an article on auto-rx as well.
You could send off an oil sample for analysis. There are posts on oil analysis services on the site as well. If you are confident that your problems stem from the oil change, it may be worth the ~$30.00, plus the two oil change costs, to run a bottle of auto-Rx through your engine and rule out the sludging issue.
Good luck and let us know how it works out.