Hello and thanks in advance for any help. I've searched for this first and didn't find any specific info so here goes. My Case 448 with an Onan B48M is burning 1 quart of oil every tank of gas. We did a valve cleaning job 6 months ago and based on some forums advice have been running 15w-40 Rotella since. Now the mower is doing this. Both cylinders test out at 120. Some posts say we should be running straight 30 oil. I am lost since I am not a mechanic. But I do know one who can do the work needed. Can someone give us an idea where to start?