The Rock oil website gives a pdf brochure:
http://www.rockoil.co.uk/cm/pdf/rock...ure_marine.pdf
There are 2 products listed in the pdf which may masquerade as a DFI oil:
"Non TC-W3 2 Stroke
engine oil
Rock Oil offer two high specification 2 stroke lubricants for use when
a non TC-W3 two stroke oil is recommended.
Both PP2 (semi synthetic) and Synthesis 2 Injector (fully synthetic)
meet the very latest international 2 stroke specifications. They are
both ashless clean burning lubricants ideal for use in high powered 2
stroke applications.
They meet or exceed the following specifications:
JASO FC ISO-L-EGD API TC"
IIRC, the Optimax DFI oil doesn't have an API spec, so I'm not convinced these products would be suitable to use instead of the genuine article.