Here is a first hand account of why silicone hoses, while great, cannot be used for oils and fuels.
Step 1: The Example.
Here is the vacuum hose from a pcv valve to the intake air manifold of a car engine. See how dirty it looks next to the adjacent silicone hose? In fact engine oil that is coming from the pcv valve is weeping through the hose.
This car has a history of venting lots of oil mist from its crankcase due to lots of blow by gases.
Step 2: Removing the Hose.
With the hose pulled off the pcv valve and the intake manifold nipple, you can see inside the hose has heavy oil contamination and also the outer surface too.
Step 3: The Correct Hose.
Fuel grade rubber hose is what is needed for this application. It is impervious to oil and will work perfectly.
For this car however an oil air separator is needed to trap the oil and prevent it from going to the intake manifold. I will cover that in another instructable.
I hope this instructable has been helpful to other car enthusiasts.