I know that Wiki says Havoline XLC but this exact label seems to be European only. What ever that is in there now is rather red. Probably will be doing a valley pan very soon. Present coolant is no more than 16k miles old. Is there a US equivalent? I dont want to cut corners here. If my current coolant is the wrong stuff how critical is it to flush it all out? I've read that some coolants dont play well together.

If it's red then it's almost certain to be an OAT fluid (Organic Acid Technology). Any OAT or HOAT (Hybrid) will be compatible with it.

The confusion comes that early 4.4's (and probably TD6's) used IAT fluid (green) but it was swapped to OAT later on in production. You can swap from IAT to OAT BUT the entire system needs flushing through first otherwise the old IAT fluid will react with the new OAT negating any benefits. For the same reason OAT fluids should only be diluted with distilled /deionised water as impurities in tap water, especially in hard water areas, can also effect the life of the fluid.
IAT fluid service life is around 2 years, OAT fluid is around 5 years so you pay more for OAT but it should last over twice as long.

as a general rule of thumb if the coolant is red, orange, pink then it'll be OAT, and it wont mix with anything blue/green.

So long as you don't mix the types you can use either really, but as already said OAT lasts longer, it's also a bit kinder to the environment and doesn't eat through paint etc.

I think i'll be worth putting some cooling system flush in the system before you drain it, because you don't know for sure if someone has topped up with incompatible coolants in the past.

Oh that clarified everything! I was 99% sure the PO used the correct coolant but I need to top it off until I get a chance to confirm and fix the leak. The local auto store was trying to tell me fluid color didn't mean anything.

In truth they're probably right, in that it's not a definitive way of telling, but I'm not aware of any blue/green oat out there. It's a pretty good indicator though.

Flushing it through isn't a bad idea anyway, should get rid of any sludge in there and reduce the chances of blockage. For the sake of a £5 bottle of flushing detergent and the extra 15min it would take.