You need a fluorosilicone-lined hose wherever the inside of that hose will come into contact with fuel, oil, solvent, fuel vapour or oil mist. That is the whole rule. Everything else is about recognising which applications fall inside it, because some of them are less obvious than they look.
Standard silicone is chemically at ease with air, water and coolant, and it handles high temperatures better than most rubbers. Hydrocarbons are the exception. Fuel and oil are absorbed into silicone, which softens the material and makes it swell. A hose that swells no longer grips the pipe it is clamped to, and it can delaminate from the inside out. Because the damage happens on the inside, the hose usually looks fine right up until it fails.
These are the classic cases, and the most commonly missed. A crankcase ventilation hose carries oil mist and blow-by vapour continuously, not occasionally. An oil return line carries hot oil. Both look like ordinary air hoses from the outside, and both will degrade a standard silicone hose from the inside over time. If a hose in your system is handling crankcase gas rather than clean air, it belongs in this category.
Any hose whose air path passes through or near a fuel component can see fuel vapour, particularly at cold start and during purge cycles. Evaporative emissions plumbing, fuel tank venting and lines that connect to a fuel-adjacent device all fall into this group. On a road car with a full emissions system, there is more of this plumbing under the bonnet than most owners realise.
Ethanol-blended fuels are more aggressive towards elastomers than straight petrol, so a hose that is only marginal on standard fuel may be clearly inadequate on an E-blend. Marine applications bring their own combination: fuel exposure plus a humid, salt-laden environment. Both are cases where the fluorosilicone construction is the sensible default rather than an upgrade.
If the hose only carries air or coolant, standard silicone is the right answer and spending more on fluorosilicone buys you nothing. Charge pipes, intake paths, intercooler boots and coolant lines are all standard silicone applications, and they are the majority of hoses in a typical engine bay. You can browse those on our performance silicone hose page, and compare the fluid-resistant construction on our fluorocarbon lined silicone hoses page. If you are not sure which side of the line your application falls on, describe what the hose carries and we will tell you.
