Two stroke oil is perfectly legal for road use. It can't be illegal, how many two stroke bikes are on the road?

Years ago sulfur in diesels done all the lubricating needed. Unfortunately modern day diesels are generally low in sulfur so lubricate far less, couple in the use of Biodiesels and you have a fuel that lubricates far less than it used to. Garages use their own additive packages to increase cetane levels, to keep the engine clean and also to increase lubricity which is why diesel is never just diesel and quality does change from garage to garage.

Two stroke oil will help to lubricate fuel pumps, injectors and the upper cylinder. It also increases the cetane level of diesel so you get a cleaner burn.
Two stroke oil is a long term preventative maintenance additive. Used over time in correct dosages (1:200 is recommended I think) you will have increased the longevity of the engine and fuel system, increased fuel economy and have a cleaner engine all round.