My old
Mk1 Leon TDI SE (110) started off on BP Standard, then went to BP Ultimate when it got released. 12m later couldn't justify the cost of BP Ultimate, so went back to BP Standard and my car felt rough. Swapped to Shell Diesel Extra which was better than BP Standard, but still wasn't BP Ultimate. Then found Millers Diesel PowerPlus - that combined with Shell Diesel Extra, was a win-win for me. Migrated to Millers Diesel Power Sport 4 and it got better still
Tried Shell V-Power with/without Millers DPS4, but didn't see much improvement over the Shell Diesel Extra + Millers DPS4 combination, ran that for 18m
MK2 Leon FR 170. Been running Shell Diesel Extra and Millers DPS4 since new. Tried without Millers once for a tank and the car felt rough no matter how I drove it. I will at some point try Shell V-Power and BP Ultimate just to compare.
My wife's Corsa 1.3CDTI (08 reg) has also been running Shell Diesel Extra and Millers DPS4 since new. The odd time she has filled up at Asda/Tesco in an emergency, the car has felt rough. Again, without Miller's, the car feels very rough.
Overall, I'm very happy with Shell Diesel Extra (and Shell Drivers Club points and money off fuel vouchers) and Millers DPS4. Have converted a number of friends and they are now using Millers. Both in turbo and non-turbo diesels and all (bar one) has noticed a difference, especially in engine noise = better.
Goot