500 miles on rebuilt 340 - NEVER had an overheat issue. Chrome thermostat housing was leaking, changed it to cast iron - then added a bottle of the Bars Leak liquid with "pellets" for preventive measure AND because I had a couple freeze plugs weeping after parking. Could the Bars Leak be causing the issue ? What temp thermostat should I run here in Florida ? And three .... water in system feels oily - but I thinf thats the Bars additive ?

Yep .... guess I'll be flushing.

Sounds like it plugged up some tubes in the Radiator

The bars stuff is oily. 180 Milodon stat w a 1/16" bleed hole(& check it suspended in a pot). Rad now may need boiling out.

I'll flush, ad t-stat and see. Shoulda left well enough alone. Why do they sell it advertised as a "preventative and conditioner" if it can actually harm the radiator ?

Don't forget to replace the weeping freeze plugs.

Agree with above, do your best to get that stop-leak cleared from the system and hope it hasn't permanently plugged radiator and/or cylinder block/head passages.

Also, that "weeping" freeze plugs could be causing the problem. If your system will not Hold Pressure, the coolant/water will boil at a lower temperature.

You know, sometimes it can be a cheap/simple fix too.

I bought my Factory AMC V8 1979 Jeep CJ-5 in 1998. I could tell the previous owner hadn't driven it much in the months/years before he sold it.

When I took possession, had to drive it 40-miles to get home. I made it 15-miles before it overheated.

Stopped several times and limped home. Thought it was a Hole in the radiator or bad waterpump, etc.

After quite a bit of diagnosis and much concern, turns out the ONLY problem was a Bad Old Radiator Cap not holding pressure.

Bought/installed one $3 radiator cap and it cooled like a champ. That was the only problem, so you never know.