Here's one for you - at each end of the block, there is a narrow strip leading from the deck surface up to the end rails of the block. Lay a strip of silicone on those areas before bolting on a head. There's no bolts in those areas, so the head is not getting tugged down there. And that area is open to the lifter valley, so there is an oil source available.
Be sure your valve cover breathers are working properly, as well. If not, the crankcase will pressurize and start pushing oil out of any weak sealing areas. Ever ran a motor really hard and noticed the dipstick was pushing up and out of the tube?
Toss intake manifold gasket end seals in the bin. Make sure those surfaces are all clean and dry (brake cleaner, acetone, lacquer thinner) and then lay on a healthy bead of silicone. But don't stop at the intake surface of the head, be sure to work some silicone into that corner and then make a light pass up onto the intake gasket. Let it set up a bit, before bolting on the intake.
Whilst silicone can be your best friend, remember it will not seal up surfaces that are already oil-covered.