O'reilly has a good deal on Rotella T6 5W40 full synthetic right now. $21.66 with a $5 rebate = $16.22. You can run Synthetic 7000 to 10,000 miles, with normal service, with hard service, just change the filter at 5000 miles. To check it out, change filter at 5000 miles and send in a sample for analysis.
Use the Motorcraft FL 1995 filter. O'reilly for $9.99, cheaper than Wallymart.
The records on this truck show the previous owner used Mobil One 5w40 or Lubromoly? 10w40.
Looks like 3000 mile intervals.
He sure wasted a lot of money doing that with synthetic. But ya know it was well taken care of. 3K mile interval and up to 5K interval is recommended for Dino oil like Rotella !5W40.
Use 5W40 in winter and 10W40 in summer. If your engine don't have the oil rail crossover, you should put one in it.
http://cncfabricationllc.com/index....rication-7-3l-sd-hpop-hpx-crossover-line.html
These engine like a lot of oil flow for the injectors.
CNCfabrication also has the HPOP hi pressure lines for cheap, $50 ea, and there better than the $100 ones elsewhere. Good to have a set if only for spare, they can blow at any time.
By the way, put your truck info in your signature.