Just goto your local factors and buy whatever their cheapest stuff is. Usually carlube, comma or granville.
Given its not a turbo, you want 10w40 semi synth, you can also use 5w40 full synth if you want to splash out, but its less of a requirement on a non turbo engine. Should be around £15 a gallon.
I use cheaper oil (5w40 synth) and change it every 5k, rather than paying 30-40quid a gallon for fancy castrol stuff and leaving it in for yonks.