My daughters 3.2 DiD only normally used around town for short runs and didn't use much oil between services recently did a 500kms trip at I presume 120-130kms /hr and the oil level dropped from high to low mark on dip stick..

I cant say if it was smoking badly from the exhaust etc but what can the causes be of this oil usage?
The engine starts very quickly when cold so cant be piston rings etc, mechanically the engine sounds very solid and no funny noises and it pulls like a train - very strong.
I was thinking turbo or maybe valve stem seals?
285,000kms on the clock but regularly serviced.
Thanks
Ernie