X15 Oil Fuel Dilution

[Hunter013]

I have 2 trucks with identical motors that around 400 hours they start to get low oil pressure. We change the oil and it runs fine for another 400 hours. We've been sending on oil samples since the issues started and it took about 3 samples before they found fuel diluting the oil. What should be the first things I look for? Injector o-rings or injectors? Trucks run well otherwise.

Attached is the sample report. I use Caterpillar sample system because they give it to use free when we buy equipment.