Ron,
In the Power Stroke Diesel (PSD) Ford recommends a 15W40 for normal use, a 10W30 if temps are constantly below 20F and a 5W30 if temps are constantly below -10F. The key is constantly below. Also CF-4/SH or CG-4/SH or above ratings are the recommended ones. Most all Diesel oils OTC will meet these specs. I use CI/SL spec oil.
Now the 5W30 Mobil I is rated SL/CF. In a diesel engine the Commercial rating preference should be listed first then the Service rating as in CI/SL etc. The Mobil 1 won't blow the engine or drop a piston out the bottom, but the first thing that he will notice is rough idle and a skipping/hesitation symptom. This is caused because the Mobil 1, lacks the anti-foaming agent required by the HP Oil pump in this engine, which varies from 500 psi at idle to 3600 psi at WOT. This is a pump to supply oil pressure to the injectors. (HEUI injectors by CAT) The engine also contains the normal engine sump oil pump for the rest of the engine. If your friend would go to an IH dealer and get a bottle of anti-foaming additive, PN CH1824392 or Lubrizol 888, he can prevent the foaming issues.
IMO he used 15 quarts of oil not rated for use in this engine. If he wants to run this for the winter, add the anti-foaming agent, and change very early spring. I personally would not use this oil in any PSD except an emergency. Gasser, great. If he has any engine related problems, as noted earlier, change oil to the proper specked oil prior to taking it to the Ford Dealer. The easiest cure for a rough running PSD is change the oil first and if the problem persists then go on to further troubleshooting techniques.
Sorry for the long post.
[ November 27, 2002, 02:56 PM: Message edited by: 59 Vetteman ]