Truck Repair: Isc cummins

Question
Hi there I have a 2001 Peterbuilt 335 with Isc engine with 250-300hp. The oil keep coming out of the pest tube.We have drill a hole in the oil cap to keep breathing. When we are on the hwy the truck goes through a gallon every 7hrs if your driving below a 100km or 60 miles per hr it's much better on oil. When we run around town it use a liter a month. someone told me he seen that in a turbo problem because when turbo pulls on the hwy steady so it burns more but in town it doesn't have enough chance to create pressure.Also someone told me about the EGR but i don't know if i have one. Someone told me the termostat because if they stay cold the motor just spits it up  so please let me know your advice
Thx Don

Answer
If that's the original engine in your truck egr isn't a issue that's pre emissions. Not sure on Canada but the states didn't put on egr til 06 I believe. As for the thermostats your oil and coolant don't go hand in hand unless you overheat Then parts break. I could see possibly a turbo putting pressure in your oil system. What I'm leaning more towards is doing a compression check because to me it sounds like rings are worn or broken. Causing compression to get in your crankcase and blowing it out your blowby.