Cumba, usually the trucks spend oil, I have a double cabin Luv Luv 2,3 and spends a lot of cooling refrigerant too, the point is the other syntoms.
In the case of compression pèrdid Increase, in the case of Vàlvulas you must rectify.
If it is the most serious case, that of a semi-support or rings change the engine tends to sound stronger, a rattle of the piston feels even in the cabin, and if oil continues to pass completely the compression of unistòn, low approx to 80 psi.
I hope that serves you