Well....pulled the valve covers this weekend to do compression testing. The compression tests came out reasonably well (about 350 -360 psi) on all cylinders, I'm guessing thats ok for a 293k mile truck. I did notice that cylinder #4 has a newer "AD" injector. All other injectors were a darker brown "AB". Can "AB" and "AD" be mixed? Can I go all AD without flashing or special Tunes? Is there something else I can check, to see what injectors I should be using?