P0128 code /Fluid Flush/ Refill

[Romeo]

Hey all, my truck is close to 80k miles and recently started receiving P0128 code. Just called a dealer in my area and they have one thermostat left in stock so I'll be going to pick that up. As I head to the mechanic sometime this week I want to also flush and fill my fluids as I've never done any of these. What do you guys advise i refill with new fluid, what fluid type you recommend I use and how much is needed so I can order online before going in the store.

 

[K0PIR]

I use Prestone Dexcool. I pick it up at Walmart. You'll need 2 to 3 gallons.