My attitude has always been to put in whatever the manufacturer's spec says. I use semi-synth as I have it changed every six thousand miles. If it was a 12k service interval, I'd want fully synth. Never too bothered by what the brand is. I don't do a huge mileage, but I tend to own cars for a reasonable length of time. The only thing *touch wood* that has never failed on me is an engine. (Apart from my first car, a Fiat Uno, but that was drinking a litre a week and I just couldn't afford to keep putting the oil in, so I stopped and one day it went poof>)
Ex 95 164 TS Super, 95 164 QV, 97 164 V6 Super – all very much missed.