Permeability to oil and fuel
The molecular structure of silicone doesn’t play well with oil, fuel, and other oil-based products. We do not recommend using silicone hoses for any applications where they will come in contact with fuel or oil.
Permeability to water molecules
Silicone is watertight, but water vapor can escape very slowly through the walls of the hose. Think of a balloon that you blow up with air and leave on a table for a week. The air escapes very, very slowly through the walls of the balloon. There’s a similar effect with silicone, and if you have silicone hoses on a car that you use as a daily driver, you’ll find yourself having to replace water in the reservoir from time to time - not because you have a leak, but simply because the water is escaping through the walls of your hose. While silicone hoses are a must for racing applications, some people don’t like them in a daily driver.