If you look in the handbook you get the specifications of the oil needed. Probably any semi-synthetic oil you can buy will be better than the handbook spec'. API-SL and B3 is pretty much as good as any semi-synth.
If you change your oil every 6000 miles, then even better.. but it should be up to lasting 12,000 miles (as all semi's).
But I found (through travelling to Jupiter and back) that most semi-synth leaves gum and varnish in the heads if you go to the manufacturer's 12,000 mile limit. I think 6000 miles would prevent that as the oil is "newer" for longer. All but the most hard-core semi-synth's break down with miles.
Having said that, Ive always run the oil to 12,000m and I've had very little engine wear with semi-synthetic. I "only just" about had to adjust the tappets once in 100,000 miles for instance.
Fully synthetic can go 12,000 miles easy. It retains its viscosity (doesn't break down as much) and has a higher detergent level, to remove gum and varnish.
Unfortunately, it costs twice as much..
Having cleaned out my engine religiously while the heads were off (head gasket) I've decided to switch from semi-synthetic/12,000 miles to synthetic/12,000 miles. My old beast is only worth a few quid now
so putting synthetic in there and changing it every 6000 would be a bit over the top.
But the better the oil and the more often you change it, the better.