I had always been under the impression that supermarket diesel was supplied from the same sources and was of equal quality to that supplied by any of the other petrol stations. Recently, however, it has been suggested to me that supermarket supplies are cheaper because they do not contain all of the additives that are present in the diesel sold by other petrol stations. Could you advise which is correct? Thanks in anticipation,
John Sullivan

The diesel fuel supplied to supermarkets does indeed come from the same sources and same distribution depots, but that does not mean that it necessarily has the same additive package of things like detergents, that help keep fuel systems and injectors free of unhealthy deposits, and of cetane improvers that help to improve fuel combustion. These additives are a bit like vitamin pills, and whether your engine needs these additives may depend on how you drive and your engine’s characteristics – some engines will run well enough on basic fuel with minimum additive levels, and all UK diesel fuels have to meet EN 590 standards, which require testing for cetane number, lubricity, and resistance to chemical degradation, but there are not any standards for detergency. All engines really need to contain a minimum additive package above what EN 590 demands, including detergents, and all respectable supermarket diesel fuel will normally contain this, but possibly at varying levels of strength.

The debate is really about whether the benefits of extra additives is significant, and whether the further enhanced additive levels in the premium diesels like BP Ultimate and Shell V-Power Nitro Plus are worth the significant extra cost of 8 to 10p a litre, which I feel is definitely not the case. I feel that the best compromise is to shop around and find a source of branded fuel (BP, Shell, Total, Esso etc.) that is as cheap as supermarket fuel, which there is in most areas, and then dose it with Millers Diesel Power Eco Plus additive, which if you buy it right will only cost you effectively around 4p a litre, and give you an additive package at least as good as the premium fuels, but at half the extra cost.

You’ve probably read me extolling the virtues of Millers over the years, and may think I’m on their Christmas card list, (I’m not!) but I’ve had countless letters over the years reporting that use of Millers (sometimes initially on a one-tank double dosage basis) has solved many running problems. On Amazon, you can find it for under £10 (with free delivery, if you are patient) for 500ml, which treats 500 litres of fuel at standard rate, of 2p a litre, or 4p a litre at double dosage. So go and buy Esso, Total or whatever at the best price you can find and then experiment with Millers at standard or double dose. If you don’t find any improvement in engine performance and fuel economy over an extended period (best try it for three to six months maybe) then drop back to the cheap standard fuel. Hope this helps, but you may have to do some mild experimenting to find what’s best for your engine. Best regards,
Doc Diesel