i did in my gen 4 a long time ago...well 2 out of the last three were synthetic, the other the dealer did when they checked it for sludge when i got my recall papers i personally love it, car runs smoother runs cooler etc, only issue ive had is my worn valve guides smoke a little more often on start up...otherwise its been great. and at a total of 15 bucks an oil change i aint complaining.
High mileage would be better than synthetic with 155k. Today's API SM oil is synthetic blend anyway. Unless you intend on extending drain intervals significantly, the 2-3 x cost is just not worth it. We have used Chevron and Pennzoil for 192,000 sludge-free miles.
no it wasnt rough before, but it accelerates smoother and quicker, the temp stay a little lower, mileage went up a few miles and like i said for 15 bucks a change it was worth it, plus i live south of houston, it gets hot and humid here so good oil keeps an engine happy (also after the mass evacuation a few years ago, ill do whatever i can to help keep my car happy )