My dealer did use Silkolene but they've gone over to another brand since my last service. Currently, I'm using Rock Oil Guardian (semi-synth. £24 for 4 litres) Only time will tell if it's any good.
I use supermarket's own brand (£6-8 for 4 litres) in my car ('96 AX diesel) but change it every 4k miles or so. I'm of the opinion that frequent changes go a lot more towards engine longevity than special formulaes.
