Not all Diesel Oils will meet the needs of a flat tappet cam. Rotella is only marginal but it will work depending complettly on the cam SPECS. If you are running a stock cam you will be fine with the Rotella, but high performance cams are going to need more of the ZDDP. When you start running after market springs with there increased spring rate that is when the modern oils of today let you down.

The two oils that I personally will trust are valvoline Racing oil and Brad Penn oil. Both have the required additive pack that will give a flat tappet cam even a chance at living. Also both of those oils have plainly stated on the label "Not For Highway Use" that is because the ZDDP clogs the cat's on modern cars, thus the reason the additive pack containing ZDDP was dropped, and reduced in Rotella. I am not familar with the Motorcraft Oil mentioned by jeffB2 but if it does in deed contain 1335 PPM of ZDDP then you should be fine.