xr burning oil why?

hi guys, i own a xr650r 2003 which after approx 2 months of owning it blew the piston and barrell,on inspection i concluded this was down to lack of oil i drained about half of a litre out,i fitted a wiseco 102.4mm piston and had the barrell rebored anyway the bike ran perfect for 350 miles (slight smoke on idle which dissapeared as soon as you set off, this was on a 68s pilot, with the 65s pilot the smoke stopped i put this down to the bike running a little rich)and then it seized up again causing piston slap knocking. after yet another oil inspection i could only drain half litre out 350miles earlier the bike definately had approx 1.7litres in (checked level instantly after a run and was approx 1 inch from top of dip stick).my conclusion is that it is the drop in oil level which is causing the bike to seize up but there are no external oil leaks and no smoke from the engine when under normal running conditions(only very slight on idle), so does anyone have any idea how and why my bike is using approx 1.5litres of oil every 350 miles causing my wallet some real pain. your help and comments will be apprechiated cheers.

you will need to know if you have bad valve guides. is the plug getting fouled with oily deposites? my Harley was and I had extensive head mods done with bigger valves and port polishing. this can also be done on any head really, and if it needs some maintenence, it's a good time to hop it up.
see if your oil is being burned through the valves. it's possible, and there's not much volume in there to spare, it will suck it down seemingly fast.
good luck.