Cat 257B blow by! Please help

I have a 2007 Cat 257 B with 652 hours. Here's what's going, I've noticed that running it a few hours here and there I'll check the oil and started noticing it lower on the dip stick every time. I don't run the machine very often. It doesn't have water in the oil or oil in the water so I didn't think it was the head gasket problem. It's not knocking & sounds like it's running fine but while it's idling I take the valve cap off and it's puffing white smoke out bad. A few mechanics thought the rings were stuck so they told me to change the oil, flush the engine with diesel & put in new oil with marvel mystery oil. So that's what I did! However I ran it for 15-20 mins not very hard today and the low oil pressure light came on! We did not have a pressure gauge but we did take the quick connect off and turned the engine over and it is getting plenty of oil to the top of the heads. I plan on buying a new oil pressure sensor Monday but does anybody have any suggestions or comments? Anything would be greatly appreciated. Thanks

Cat 257B blow by! Please help