[spinlondon]

To check whether the valve seals are leaking (without removing componants), you have to get up to speed, shut the throttle down and decelerate with engine breaking, then open the throttle up again. If a lot more than usual smoke is produced, then it is the valve seals. Obviously the more you use engine breaking, and the more leaky the seals, the more oil will be burnt.

It is possible to replace the valve seals without removing the heads on most vehicles, but only with specialist tools and a compressor.