I agree with J94 in that Castrol is not what I call a "true" synthetic. In most areas of the country, Mobil 1 is the only true synthetic oil available over-the-counter. What I call a true synthetic is oil made from Group IV base oils (polyaphaolephin...PAO) and Group V base oils (esters and others). Other true synthetics are Royal Purple, Redline, Amsoil (except their XL-7500 series).

Castrol Syntec (except maybe the German imported stuff sometimes for sale in Canada) is made from very highly refined petroleum with the legal right to call itself "Fully synthetic"...this is Group III base oil. Pennzoil & Quaker St. syn, Shell syn, Chevron syn, Petro-Canada syn, and all the rest are also GR-III.

Syntec is good oil, not quite as good as full synthetics such as Mobil 1, so everyone has to decide if the extra cost is worth it based on their driving habits. If your engine is operated occasionally under severe conditions I'd highly recommend using a full synthetic over Gp IIIs, blends or Dino.

I don't think synthetic oil will do much relative to "flushing" your engine. In most cases when changing from Dino to synthetic you probably don't need to flush the engine but if you "feel the need" or know your engine is gunked up, then flush your engine using a "specialty" chemical that's manufactured for that purpose. There's several on the market that will do a good job but the one I'd recommend is CFL OIL sold by Lubriplate. Put a quart or quart and a half in, drive 300 miles and you'll "puke" your engine almost spotless. <img src="/forums/images/graemlins/cheers.gif" alt="" />