This question comes often. I was about to reply to it, but the thread was locked because an infamous blog was mentioned.
For a flat tappet, you need a thick base oil. Any CK-4 15W-40 is an excellent choice. It sounds like you have an oil-pan heater; so, you should be OK in Colorado. In properly certified modern oils, the ZDDP content (phosphorus level) does not matter whether it's a flat tappet or not. Amsoil HD ACD 10W-30/SAE 30 is a monograde (no viscosity-index improver) with a very thick base oil, and it would be an excellent choice as well, except for the price. Some 5W-40 HDEOs are halfway decent choices, with a moderately high base-oil viscosity, if you don't have an oil-pan heater or a block heater, but they won't be as good as a 15W-40 or the Amsoil HD ACD 10W-30/SAE 30. See my spreadsheet linked below for the base-oil viscosity. The base-oil viscosity is listed under the BO DV150 column (cP units). You want a high number. Estimated VII content and base-oil viscosity of selected oils
Originally Posted by Mad_Max_Sam
I own several 'flat-tappet' hot rods, including my mighty Cummins 5.9 (solid cam), and so I've got a couple questions... I talked with the tech folks at Shell and they claim their Rotella T4, 5 and 6 all have the 'flat-tappet' package, so I've been running Shell Rotella T4 10W30 in my hot rods, ... Meanwhile...I am still trying to determine what to run in my Cummins ... I may have to resort to the expensive mail order oils like Amsoil... Would appreciate any thoughts/recommendations. Cheers - Sam