My cars manual says it's fine on 'normal' grade petrol. I can understand that the higher grade is better in fast/sports cars but a normal car the only benefit would be to help keep it cleaner inside but I don'tthink even that would make much of a difference.

I can say for definite that in the case of BMW, HJ is correct.

Taken from here : www.bmw.co.uk/en_GB/new-vehicles/3/saloon/2011/eng...l

Rated output: BMW recommends the use of super unleaded 95 RON fuel. Unleaded RON 91 fuel or higher with a maximum ethanol limit of 10 percent (E10) is also permitted. The performance and fuel consumption rates listed are based on the use of RON 98 fuel.

With petrol, it's relatively simple: premium petrol is higher octane (usually 97 - 99) than standard (95), which some engines can take advantage of by advancing ignition timing, upping boost pressure to get more power / more economy.

But with diesel it's a hugely contentious issue, and the oil companies themselves don't do anything to clarify the situation by giving any easily-accessible facts about the actual specs of their fuels.

For example, Shell V-Power Nitro+ diesel is, according to Shell: "formulated with a powerful cleaning agent, designed to prevent and remove performance robbing engine deposits that can be left behind by non-premium fuels. Our advanced formulation is designed to work under the extreme conditions of the fuel injection system, breaking down and cleaning-away harmful gunky deposits."

Fair enough.

But Shell's own blurb for its regular-priced FuelSave diesel is: "FuelSave is designed to help improve your engine’s performance, and enable your vehicle to run more efficiently. It contains a powerful detergent, designed to help clean up and control harmful fuel system deposits in fuel injection equipment in modern diesel cars, helping to restore engine performance."

Isn’t that what they say about V-Power Nitro+ too? So does the latter have more detergent? A different, better detergent? Magic pixie dust? Shell doesn’t say.

A while back I managed to track down Shell’s own material data sheets on both its FuelSave and V-Power diesels that are retailed in the UK. The data made interesting reading:

Cetane number (typical value): 51 to 53 (V-Power); 53 to 55 (FuelSave)

Cetane index (typical value): 51 to 53 (V-Power); 53 to 55 (FuelSave)

EVERY other parameter – from Sulphur content, flashpoint, carbon residue, ash, water, particulate matter, lubricity etc, was EXACTLY the same between V-Power and FuelSave (for the sake of clarity, the V-Power specs are for the old V-Power, not the current Nitro+ branded stuff).

But the point stands: according to Shell’s own data sheets, the bog-standard FuelSave diesel has a slightly higher cetane number than the costly equivalent. Cetane isn’t everything when it comes to diesel, but a higher cetane value is generally accepted as a good thing.

And if they claim that both diesel fuels do a great job of cleaning up deposits, but don’t say exactly what’s different between them, and one costs nearly 7% more, which one are you going to use?