Chevron's Texaco Techron HD is designed to remove injector deposits from engines, caused by the rise in biodiesel and higher injector pressures at Euro 6.
The claim is that the fluid returns engines to as-new in terms of fuel efficiency.
Chevron is currently rolling out Techron HD to UK truck operators but this is the first time the product has been offered in the bus and coach industry.
"Our customers frequently report loss of performance and fuel economy from modern diesel engines, particularly on intensive stop-start inner city applications," states oil industry expert and Optimum managing director Cliff Thrussell.
"Techron HD – which is available either as a one shot clean-up treatment, or as a continually dosed cleaner – returns engines to health and can maintain them at peak efficiency for a modest investment," he continues.
"We are delighted that Optimum, a true expert in the needs of passenger transport fleets, is going to work with us and we wish them every success," he adds.