Nowadays I use only a few sealers for gaskets, in fact I use one instead of gaskets a lot. This would be "Right Stuff", like silicone but far superior.
On a paper gasket like the front cover or the water pump or the thermostat housing, I use "High Tack", a Permatex spray can. The trick here is to have the gasket and the parts virtually sterile by wiping them lightly and repeatedly with laquer thinner. I grasp the gasket with hemostats and spray it both sides with three quick coats of High Tack, then hang to dry a bit. Then stick it to the part carefully touching it as little as possible, put the parts together and make sure your bolt threads are spotless, no dirt being shoved in. How you tighten the bolts is important too, don't crank down on them one at a time!
For oilpans I don't use gaskets, "Right Stuff" is awesome here! Doing this can be tedious, but let me tell you there is not ONE drop of oil under my race car EVER! The trick here is clean parts, a nice even bead of "stuff" and then put the pan on but don't tighten the bolts until it's cured. Give it a week, then remove one bolt at a time, put "stuff" on the threads and just snug the bolt in. Done right, you've created a nice big thick rubber gasket that bonds the parts together.
Rear end cover: same thing, "Right Stuff" in a 1/8" bead all around it, circle the bolt holes, and set it aside for a week. Then bolt it to the housing, put the bolts in just snug but not real tight. Mine does not leak at all, and I can take the cover off any time and put it right back on with no further gasket fuss. It's been off four times this year, every third race the rearend deserves a quickie gear inspection and new lube...
Some of your gaskets need no sealer. I do the intake and exhaust manifold gaskets dry. My Valve cover gasket is cork, I use 3M Trim Adhesive aka "gorilla snot" to super glue the cork to the valve cover, here again clean is good for adhesion. This allows easy removal of the valve cover to check valve lash often (before every race in my case) and I just wipe the surfaces (unglued side of cork and head surface) clean and slam it back together. Yes this will "seep" a tiny bit of oil, not a leak at all but a visible dust collector that wipes down with a rag... had I some hydraulic lifters and didn't want to peek into the head now and then I'd "High Tack" both sides of the cork and put it together forever.
This could go on forever, basics are use good sealer, just enough but not too much, clean all parts and bolts, use your head, take your time. I hate oil leaks. Especially where they will get you in big trouble, like a delivery truck making a mess out of somebody's driveway, my pickup sitting in my own driveway more than not, or a race car on a track where five dozen other cars are counting on having good traction tonight....
