Motor Oil

Is it advisable at oil changes to add an oil treatment product? My 3800 S2 has 135,000 miles on it.

Welcome to AF thunderhead.

Oil additives aren't necessary. If you want to try something, stay away from ones with solids in them (like PTFE) as their effectiveness is in question. Stick with pure oil products. Most oil additive claims are hogwash as far as increasing mileage, performance, etc... To quiet noisy lifters, Lucas is a decent one. If you develop a small oil leak that is hard/costly to repair (like a rear main seal), CD-2 engine oil stop leak is a product I've used with success.

No leaks. Have you heard of a product called "RESTORE"? I`ve used it before in my SUV and it`s a pretty thin concoction. They of course claim it`s the best thing since bread. A lot less thick than STP or motor oil. Just don`t know if i`m really getting any benefits or not.

just use a "high Mileage" oil

Restore is a good product for band-aiding a tired/worn engine, but I would suggest against using it in an engine that's running fine. It has solid particles (it's in the bottom of the can, that's why you need to shake the crap out of the can before pouring it in) that help fill the scratches in cylinder walls to improve compression in worn engines. It only lasts maybe 2 oil changes, so you have to use it regularly to reap the benefits.