Just to clarify, I should have mentioned that yes, I did check for obvious
signs of leakage ... yes, the tailpipe is coated with soot ... yes, I only
check the oil when the engine is cold, and yes, I do know how to operate a
dipstick ;-).  And I only see blue smoke if I really gun the engine, and
not with normal acceleration/deceleration.

In terms of the solutions, I do like the economy of the frequent oil
change/top-up answer, although this might be a hassle if I have to keep
stopping every few hundred kilometres on the highway.  Just to be on the
safe side, and to find out if it's a serious problem, I think I will be
taking it in for a real diagnosis.

Thanks to Patrick, Don, Joel & Sean for your responses to my query.

Cheers,

Katherine