How come I get oil leaks at the corners of my intake manifold?
Most rebuilt engines have been machined in one way or another. Two of these places are the block's deck surface, and the head surfaces. Every time material is removed from these two spots by "decking the block" or "re-surfacing the heads", it lowers the heads on the block which widens the space between them. In turn, this causes the intake manifold to sit lower on the block.
When you try to stuff those rubber or cork end seals that come in the gasket sets underneath your intake manifold ends, it causes the manifold to ride higher than before, which means you either need real thick gaskets between the manifold and the heads to take-up that space, or you need to have your intake manifold end surfaces machined down a bit to match what was taken off the block and heads to locate it back to where it originally was.
The easiest thing to do to avoid any of these headaches is to just throw out those end seal gaskets, whether they're cork or rubber, and just use a bead of good silicone instead.
After more than 3 decades of doing this kind of stuff I've found that Permatex "Right Stuff" silicone is by far the most brutal and most durable silicone there is. Yes it's a bit more costly than your run of the mill RTV silicone, but it ain't worth saving a few dollars over having an oil leak that you're ultimately going to have to fix again anyway. Just do it right, once... and be done with it! Like I always say about engines... you pay for strength and quality. Well, that also applies to most other things too, including silicone sealer.
For those of you who say you should always use the end gaskets; I can't think of anyone more educated on intake manifolds than Vic Edelbrock, who openly recommends in all of his intake manifold installation instructions that you toss out the end seals and just use silicone instead. But hey, what does he know? LOL!
Keep in mind that the oil in the lifter valley is just splash and not under any pressure, so it's not like oil is trying to spray out or anything. Silicone is by far the best sealer instead of those replacement end seal gaskets.
Another big problem you can run into when trying to use those end seals are vacuum leaks between where the intake manifold ports seal against the head ports, as well as potential water leaks between the intake manifold's water passages and the head passages. It's best to just not use those end seals in the first place and just go with the silicone to avoid any of these issues.