I'm not positive about the antifreeze. It may have a lifespan and you could find that out on Google. However, an expansion tank can last 100 years or 100 days.

To check the small diaphragm tanks, the system pressure is reduced to zero. A bicycle valve is then placed on the valve on the bottom of the tank. It should read about 13 pounds. If not, add air. If the diaphragm is broken, there will be water in the tank beneath the diaphragm and it will slosh around. If your relief valve is blowing off, you could have a leaking pressure reducing valve, a bad expansion tank, or a pinhole in a domestic water coil.

Ususually a pinhole in the coil or leaking pressure reducing valve will cause the pressure to rise and blow off, even when the boiler is cold. You should be able to isolate both of these and see if the pressure stops climbing. Isolate your cold water supply to the coil and boiler and let it sit overnight. If it blows off, it's one of those two things.

A bad expansion tank is usually associated with the boiler blowing off upon a rise in temperature. The pressure and temperature rise proportionately.

The fact that your boiler is oil fired has nothing to do with the problems. Antifreeze can last ten years if it's maintained faithfully. Concentration needs to be maintained at around 30-35% and ph at 8.5-9.0 and adjusted as necessary. Freeze protection is checked with an optical refrectometer, ph is checked with a digital ph tester for accuracy.

Antifreeze is hard on water handling components in the heating system such as valves, expansion tanks, air vents, etc.