I'm going to run one of the two oils. I'm leaning towards royal purple since it's down the street at oreily for the same price as mobil one. After it gets 3000 miles on the new motor I am going to switch it to royal purple or AMS.
Use Ford Semi-Synthetic, change it every 4,000 miles or so. Best $24 you could spend. I am an Amsoil dealer, but always recommend changing the oil more frequent. I have plenty of good stories of guys that religiously change oil every 3k-4k. Matter of fact, Amsoil has a bulletin out recommending that 6.4 owners NOT listen to manufactures spec on oil changes (that's 7,500).
So, my vote is the $24 Walmart Ford Semi Synthetic oil change at like 2/3 the frequency that Ford recommends.
It's not a really scientific opinion, but I am not impressed with Royal Purple versus Mobil 1.
And it has nothing to do with engine oil, but I will explain:
The rear in my '01 had a slight whine on the highway, did it brand-new, under very low load, almost coasting. Did it for years. One day, I decided it was time to paint the diff cover otherwise it was going to rust through.
Removed, painted, reinstalled. Only synthetic 75w140 I could find locally was RP at the time. So, in it went.
The whine was still there, no change.
Cover developed a leak (my own fault) so I had to pull it again. Figured, try Mobil 1 this time.
Whine is GONE, and has been for 5 years.
This shadetree mechanic isn't going near RP ever again.
Either use the Ford stuff, or go with Mobil 1.
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'74 F250 Highboy FE390 deceased! I've been wrong before, I'll be wrong again. Just wait and see. ®
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