That is probably about as bad as dishwasher tablets. It's like people who put copper slip on brake piston seals - back of the pad yes - seals no! I spend a lot of time assessing materials and their suitability for applications and it truly shocks me sometimes when you discover how unsuitable a material is for a particular application.
The issues with vehicles is that they need to do so many things, they need a coolant system and associated components that can withstand extremes, they need the gaskets, seals, metals everything going on in there to be happy to be bathed in a slightly acidic soup of water, ethylene glycol, nitrates, silicates etc. If you look at the chemicals used and their impact on materials you would probably not want to use it. But all parts will be designed to withstand this soup - add hydrocarbons like diesel and white spirit (more or less highly refined kero) to your system and you now have a problem.
If you look at the rubbers used for different seals in a vehicle you will see they are almost always unsuitable as a 1 for all material. Brake seals are usually EDPM, which is great for weather resistance, low temperature flexibility and heat resistance but will be destroyed by petroleum products, oil & solvents.Then take Viton (Flurocarbon rubber) which is often used as engine/gearbox oil seals - it is one of the best at abrasion resistance which is ideal for moving shafts but it is not actually the best by far, the best rubbers for dynamic use or abrasion resistance are not suitable for use with oils! Viton doesn't actually like fuels much, happy with oil and grease though, for fuel you want Nitrile however, it doesn't much like a lot of heat (about 100°C max) and doesn't like being exposed to the weather or ozone even then there are better rubbers for use with fuel but other parameters let them down.
I like all these technical details and I find they are useful to know when reading the make up of an O-ring or seal as you know how well or not it will work in your given application.
The radiator is almost certainly FUBAR anyway, unless you are unlucky and it is a oil gallery to water jacket leak on the head gasket. The engine is obviously going to need coolant and oil change (I'd think about a flush too if water is present) You are going to need to take pipes and what not off anyway I would drop the rad now, force water into it while blocking the other side, and see if water starts to pee out of the oil cooler ports - if so carry the rad straight to the wheelie bin and start the clean up. I would do a rough clean up first, removing the stat will help, force water through the engine with a hose while adding a little soap, I would even be tempted to make a bypass pipe where the radiator should be, so you can fill the engine with clean water and a little soap and run it for a minute, then dump all the water, flush it, and repeat, think of it as a dishwasher cycle - the oil pipes should not pass any oil as they only flow when there the oil needs cooled and the oil stat opens, it is a slight risk as you may end up with oil shooting out, but it should be fine. Remember to make sure there is some oil in the engine first - is there any sign of water in the oil?
