Soon we were chugging through the city's ragged outskirts, pillars of diesel smoke barreling from the engine, the sun blotted out by our industrial-age progress into the raw heart of Tanzania.

With a cough of blue smoke the diesel engine fired up and the driver wasted no time in crunching it into gear and thrusting it out amongst the blaring horns of midday London traffic.

This indicates that smaller drops of L30 are more than those of diesel, spray quality is good, and engine smoke reduces accordingly.