I've run several cars that were meant to run on semi-synthetic oil with fully synthetic oil instead. I never mixed the oils; I did a full engine flush and drained the old semi-synthetic oil, then replaced with new fully synth oil. No problems at all. I've done this on a Rover K-series engine, a Ford Endura (HCS) engine and a Saab B204 engine, with no issues. In fact, many recommend to go against Saab's spec of semi-synthetic oil and use fully-synthetic instead as it is more resistant to carbonising in the sump next to the hot exhaust manifold, which prevents sludging problems. For similar reasons, oil-cooled turbochargers are less susceptible to damage when fully-synthetic oil is used.

Edited by robjamac on 04/11/2010 at 23:24

If I remember correctly, Halfords Oils were made by Comma. If you go to the Halfords website, you can download the MSDS info for any of their own brand products. On the form it will tell you who made the product for Halfords.