Fuel additives not exempt from duty

[Tony]

I'm sure it used to be banded about that fuel additives (injector cleaners, white spirits etc) were duty exempt as "additives" up to a certain percentage.

But I don't think that is the case, see section 2.3:

http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageExcise_ShowContent&id=HMCE_CL_000205&propertyType=document#P97_7781

Fuel additives

Products such as fuel system cleaners, injector cleaners, fuel conditioners and biocides are considered to be fuel additives and are liable to excise duty. Any product that is added into the fuel supply of vehicles either via the filler cap or via various parts of the fuel system, for example the carburettor, is treated as a fuel additive.

The duty rate is determined by what sort of engine the product is designed to be used in. Therefore products designed for use in a diesel engine attract the heavy oil rate; products designed for an unleaded petrol engine attract the unleaded petrol rate; and products designed for use in leaded petrol engines attract the leaded petrol rate. Products designed to be multi-purpose (that is, for use in any kind of engine) attract the unleaded petrol rate.

Any product that is 100 per cent hydrocarbon oil and is used solely for lubrication is exempt from excise duty.

[Keef]

The only fuel additive I can think of that is entirely for lubrication is two-stroke oil. Pretty much everything else ruled out!

I wonder where  the original, what appears to be folk law, started and what it's based on?