Synthetic oil
 

Anyone know if i can run amsoil 20W50 synthetic in my 97 FPS. They claim 25000 between changes. Want to hear some feedback.

you can run it but you cant run it 25k miles. these engines are very hard on oil and it breaks down quicker then most. i would recommend scheaffers oil over amsoil

I ran some rotella T6 synthetic and went about 5k miles on it and the motor seemed to like it. It started better in the cold and ran smoother but that stuff is expensive compaired to rotellaT and i only run that 3k miles

It is a CI-4+ rated oil so it can be run. If you need the 50 weight, it is an option. As far as the 25k interval, if you have over 100k on the truck, that drops to 15k intervals unless you are doing oil analysis.
The diese oils(except OE) can go 3x the OEM interval, but in your 97 I'm guessing that's probably still around 15k?

how far could you run some amsoil with a motor that has 337,000 miles on it?

Well if you go off the recommendations, it's 3x the OEM interval. BUT..several things I would consider. Is that motor using or leaking any oil at this time? If so, it probably won't get better and may get worse? With that many miles I would consider doing an engine flush or running amsoil for a couple shorter intervals(7-8k) to do some cleaning, then you can run longer.
I like to recommend doing an oil analysis as well. The best time to take one is with your current oil to see how things are doing currently, then again some point down the road.

Several things to consider but no reason you couldn't go 15k if everything else ok.

it uses oil but thats because the injectors are all screwed up besides that, the engine is healthy

I would not run any oil over 5k without some serious testing.

much agreed