Stanglover is right on the money when he talked about using Permatex's "The right stuff". I am an engine assembler for a living, and , I may not have seen it all, but I've seen a lot of different products used to seal up various engine components. The right stuff is definitely appropriately named. As opposed to auto parts, or hardware store tube RTV, ( which cures to a soft rubbery and not-so fuel resistant viscosity) , "The right stuff "will cure to a denser, fuel and oil resistant form. Since the topic is about intake manifolds, I should add that I have found that using this product, and never using a cork or rubber end seal across the valley under intake manifolds, I get 100% sealing, no comebacks, and customer satisfaction by doing so. I use the 6" caulking gun at work and have gotten to be quite a "pastry chef" with the way I handle it, working a tight zig-zag along the edge to a pre-determined height ( intake mocked up ahead of time with gaskets to determine gap to be sealed ) so that it compresses but doesn't squish out unsightly. It comes out so nicely that I never have to smear my finger along it, as I see done my a lot of do-it-yourselfers.
Point is, there is a "right stuff", and a right way to do it as well.
In addition, the general use of silicones and RTVs is grossly overdone by most enthusiasts. I do use the "right stuff' on sealing oil pans, usually on the end "crescents', but much of any engine can be sealed with Gasgacinch and / or the liquid brown Permatex. Many fibre intake manifold gaskets used on racing engines will use white grease sparingly around intake ports as well.
