Good question. Are you using anti-freeze or not? I have been using Pencool 3000 because 2000 says it’s for Diesel engines as you noted. However, now that I have read the fine print, it says 2000 is for “Water Only” systems while 3000 is for systems using glycol coolant. So, I am going to call or email Penray tomorrow and see if I can talk to somebody that can give me an answer. I’ll let you know what I find out.
I have also used Napa Rust inhibitor and Water Pump lubricant which is easier to find.
Some others may chime in.