I would say if you have been running petroleum oil for 108K miles then your engine will have a lot of build up on the ring grooves and ring top/bottom surfaces. They need to be cleaned and synthetic oil alone will not "clean" these.

Nor will a flush. Any brand of quick flush will NOT remove crap that took YEARS to build up. Some flushes such as Amsoil's, do help drain the old oil out, are fairly harmless and don't break the bank, but will not remove thick, well adhered varnish.

Most people don't notice the gradual loss of piston seal because of this build up. I have found a product while hanging out at bobistheoilguy.com called AutoRx....which is NOT an Amsoil product, that simply works. Damn expensive at $25, but it actually works. There are few, if any engine cleaners/additives that actually DO as intended.

With my 238K mile turbo - that generally runs really well and consumes hardly any oil - I did a high mile treatment with AutoRx and was totally AMAZED - the car makes boost way better, doesn't even come close to pinging full throttle, full boost with midgrade gas (used to have a slight ping even with premium)

Oh well I don't currently sell the stuff, so this isn't an infomercial....but as good as it is I am going to start selling it!!!!!
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Paul's Amsoil and other lubricants