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I've bought a diesel 1988 Toyota Toyoace 2.4 Litre 5 speed campervan. In good mechanical condition with engine reportedly having been reconditioned about 40,000K's ago. Driving home, first 200 k's on Mobil diesel. Tank was close to empty so filled with BP Ultimate diesel. I was surprised at the improvement in smoothness, noise level, performance accelerating through the gears, and performance on hills. Am I just imagining this?

We can think of no reason why there should be a marked difference in performance. Generally, the fuel companies all obtain their fuels from the same source, so the fuels are fundamentally the same (with the exception of Gull). BP do add additives to their diesel, but this is mainly an anti-foaming agent and cleanser which would not have a marked impact on short-term performance.