It has two jobs, raising the boiling point of the coolant and protecting the system from over pressurization. I would be shocked if any our techs ever tested a cap for leakage… Or if a $7 Stant cap wouldn’t be able to pass that test with flying colors.
My 3650 actually has a Lev-R-Vent cap and a Stant thermostat in it, both automotive. Upper Gates hose is from a Cobalt, I believe the lower is from a Civic. And a Goodyear automotive V-belt. All still in service without a single failure.
With my very old Nuffield 465 I had to mix and match parts to keep it running as most maintenance type parts were no longer available.
What comes to mind when I suggest caution is a thermostat situation posted on this forum. The tractor was a new Kubota whose thermostat was controlling two water passages. The owner had bought a generic NAPA stat and now was having problems with over heating. In selecting the NAPA stat he did not understand the cooling system design on his tractor.
This is the example which comes to mind when owners start deciding what will work without having the background to make such a decision.
There are many members who are quite capable of making such decisions but there are also those owners who come here for help who are still learning
Dave