Castrol Magnatec has had a bit of a bashing, apparently it's a 'cracked' mineral oil and not a 'true' semi synthetic - whatever that means....!
Personally I've never had a
problem with it, albeit on fairly short oil change intervals, 6000 miles or so.
However you can buy a good fully synthetic oil for less money than the Castrol 'Semi Synthetic' -4 litres of Fuchs Titan SuperSynth (how about that for a fancy name?!) for 17 quid in GSF
-Castrol normally costs 25 quid in Halfords!
The Fuchs oil was recommended by Opie Oils, who seem to be a fairly trusted oil supplier. They also recommended Motul as the top choice, but it's quite a lot more expensive.