red RTV silicone is not recommended to seal a motorcycle crankcase, its not tacky enough and slips out of the joint too.
Correct sealants are available, I used to buy them from Karachi marriot road. Threebond 1104 and 1209, they cost a lot but pay off in the long run, they are nonhardening, extremely tacky and can bear thermal expansion too. (thats what the factory used also)
For gaskets - if you buy genuine gasket made from good material they would be good, if they have become dry - put them in a bag along with a wet towel - the paper will absorb the moisture back and become new again.
For oil seals - firstly always use good quality oil seals, secondly note the seal, if it has radial grooves in which are backwards it will leak no matter what. Always use proper oil seals, if you cannot find them at your favorite motorcycle parts ki dukaan, go to a specialty oil seal shop (there are about 10 in karachi) to buy branded and quality items.
If the shaft you like to seal is grooved it wont seal. repair the item and polish it up with. Lastly always put a little grease in the spring cavity of the oil seal to ensure it does not pop out, and slowly press the seal onto the shaft and finally squarely seat in its boss. - large sockets or pieces of pipe are great for this.
If you face crankcase leaks often - check the flatness of the casing edges, smooth out with a pane of glass and emery paste. And be sure to use proper screws to tighten your engine cases, the cheap phillips head (+head) screws are garbage and require an impact driver to loosen and tighten, allen (socket) head screws are far more strong, pretty and their tools dont slip either.