Re: Error code 1391
I think the 1999 4.7 has a single overhead cam on each side. That would mean NO lifters. So the clatter you heard may be engine knock brough on by a timing issue which could be related to a cam or crank position sensor.
Changing the spark plugs may have improved firing if the old plugs were in very bad condition, but not sure either would produce engine knock. Your error code may be right on with the position sensor, but then again sometimes an error code only points us in a certain direction.
10 W 40 oil in an older high milage engine might not be so bad, but if you live in a very cold climate, that thick oil when the engine is cold could make for hard start ups and a lack of a good oil supply during warm up.
Please follow up with your progress. That could help others along the way.