My radiator cap has green sludge around it. Any ideas? Oil is normal 

Edited April 17, 20178 yr by 01cummins6
Mis typed

When was the last time you flushed the system? Any overheating or lack of heat?

Been awhile no over heating and heats fine

flush your system

18 minutes ago, Me78569 said:

flush your system

I agree.  Mike has an step by step process written up in the Article Section that is very good. 

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/cooling/cooling-system-flush-r332/

What's the best coolant

cheap and green is my method.

Is Prestone coolant flush any good?

I have not used any coolant flush products at all.  I just flushed out using garden hose and good old tap water in the radiator (thermostat removed) and the other end of the radiator sticking out the front of the truck.  After the flush I opened the bottom drain again and ran a few gallons of distilled water through it.  I just used the Cheap Super Tech from Wally World, there is no need to overpay for the engine coolant.  Read through Mikes article he addresses a lot of your concerns.

I'm using 

http://m.oreillyauto.com/h5/r/www.oreillyauto.com/site/c/detail/AFZA/G05/N0496.oap?ck=Search_N0496_-1_-1&pt=N0496&ppt=C0059