Just and idea and I know it sounds kind of stupid, but back a few years we used to add soluble oil to the coolant to lubricate the water pump seal - Castrol still sell the stuff in small plastic bottles.
Now seeing as eventually the "O" rings will fail in my heater core, will adding soluble oil to the coolant tend to keep the "O" rings supple, pliant and 'good as new'?
Denis

Depends on how often you run your heater? Only opens up when you run the heater in the car. Over summer (and I assume you are in Qld) you wouldn't even think of it. I run my heater with all the windows down once a month or so over summer for about 10-15 minutes. That way the rubber seal is getting some fresh fluid past it. But would be good to hear if that castrol lubicate works or inhibites the coolant.

Heater runs all the time, does not have a heater tap on it so makes no difference on how often you run it. When the heater is off all the car is doing is redirecting the air from the heater core around it

Also the seals on teh original fit are not rubber so again no use in any lubricant.

Mitsu hasn't used taps in the heating system since the Galant, which predates the Sigma! So its a long time ago! lol

Did some more research and found that soluble oil is bad news for anything alloy....so that rules it out unless I want a porous cylinder head.

Soluble oil, heater taps. lol Get up to date, fellas! We no longer drive HR holdens with solid steel engines and water pumps that squeak! lol

If its the soluble oil im thinking of, turns white in water, its the concentrate for machining coolant/cutting fluid? Designed to keep workpiece temps down and aid cutters in metal removal when milling/drilling/turning on a lathe?

Also any mineral oil will perish the rubber o rings.