I buy what I can get cheap as long as it's the correct spec and a brand you can rely on. Last year I had Elf oil (£20) and a blue print oil filter, blue print is owned by Bilstein I think, This year I took a punt on CEPSA oil, I'd never heard of them but turns out they're a major Spanish oil producer, (was £13.50 on Amazon, now about £35 and not in stock) and I used a Fram filter. Both combinations worked fine, it runs smooth and quiet, both oils were 5w30 A5 spec, you'd be hard pressed to tell any difference between them.