I'm wondering if I should be using a engine cleaner periodically and which is best. I've read about turbo sludge and such.

I have a 2001 A4 1.8T that's GIAC chipped with 48k miles and have been using 100% synthetic oil from day one.

 

You should be fine. Make sure that you use the big filter from Mann or Bosch, not the smaller original OEM. The larger filter adds almost 1/2 QT to the amount of oil circulating

 

A good full-synth oil like Mobil1, Castrol, or Elf will already have all the detergents your engine needs. You shouldn't need more if you are diligent with your oil change intervals.