You would be hacked off if you sent your 850T5 down to Tesco's to buy the beer and it came back with own brand French Lager ! So why when you go to Halfords / Factors do you buy 'own brand' oil of the wrong grade for your beloved 850 T5 !
The Manual / Volvo says that the 850 T5 in a temperate climate should use a quality 10W40 semi (i.e. Shell Helix Plus, Valvoline Maxlife, Texaco Havoline, Castrol whateveritis.)
The later cars (post '99) are designed to run on a quality 5W40 fully or a quality 10W40 semi.
So use a quality branded oil and change it at the recommended intervals, if you are feeling 'flush' (excuse the pun ) change the oil at half intervals.
Changing the oil more often than half intervals, when using quality oils (unless you are going on a racetrack) is not necessary.
Using a flushing additive or a flushing oil is a very good idea, especially on older cars or cars that do alot of short journeys. However DO NOT use fresh synthetic oil especially on it's own as flushing oil as this would be pointless ! (they are designed not to take on board the crud, petrol, dirt etc that is inside your engine and do this for 1000's of miles, so it will probably just leave all the crud, petrol, dirt etc in your engine !)
Regards,
Hamish.
www.volvotuninguk.com