Quest

For diesel (opacity test) yes, it is somewhat governed by the driver whose performing the test for you.

Mind you thou: 2 things come into play: (a) the most recent ultra-low sulphur diesel that WA is already mandated (available from most WA gas pumps along the highway) are very, very low in smoke, which in turn, can be translated to "low opacity"

(b) tuneups. If your diesel engine hasn't receive a proper tuneup in over 60,000kms or so, might as well do it (remember to drain and replace the fuel filter, replace the air filter as well) and make sure that your are not squirting too much fuel during acceleration (which leads to excessive smoking).

Q.