In the past 2 year we have experienced a problem with lifter rollers on our work trucks. The roller on the bottom of the lifter is egg shaped and the axle bore is wore out. we have yet to determine the actual cause of the problem. The first truck that came in is a 2006 with approx. 200k on it and had a very faint ticking noise on the passenger side of the engine. After doing a compression test we found cylinder #3 to have low compression. We removed the valve cover and rolled the engine over and found that sometimes the exhaust vale would not open all the way. We removed the head and removed the lifter and found the roller was egg shaped. We also found the roller was loose and wobbly on the axle. We inspected the cam and found approx. a .010 deep roller width groove around the cam lobe. the only thing we could come up with was the roller got stuck and wore down a portion of the roller, then started to spin again. We also noticed valve marks on the top of the piston. Just enough to leave an imprint in the piston but not deep enough to catch your fingernail on. The only explanation we had was that the lifter would ride on the worn portion of the roller while the lifter was pumped up and then the roller would spin to the high spot when the valve would open giving it to much lift. There were no broken valve springs or bent pushrods or rocker damage. We figured this was a rare, unheard of thing to happen and I even spoke to our dealer about it and they claim to of never seen this before. We replaced the engine and moved on. Approx. 6 months later we had another truck of the same year and approx. mileage do the exact same thing on a different cyl. We also replaced that engine. I now have a third engine (same year and approx. mileage) that I think is having the same problem. We have not removed the lifter yet to confirm but the exhaust valves in cyl. #2 are not opening all the way and the pushrod and rocker arm is straight. Just wondering if anybody else has seen this problem. We use a quality brand oil and service our engines at 7500 miles. Any help would be great. Thanks.