My opinion for what its worth.
Application of most RTV sealants...as little as possible. Too much and it squeezes out on the inside, it will often dislodge and make its way to the oil pump and possibly block the pick up.
Permatex Blue....dont like it at all and quit using it years ago. That said if I had it Id use till its gone. For nooks and crannys on the half moons on its fine. Less is better though.
Seems very prone to the above problem. Synthetic oils seems to break the seal it makes easily. Same with exposure to fuel. It just doesn't seem to hold up. If its somethign that is taken apart often I guess it has it place.
Permatex Ultra black is my go to product.
Its more oil and fuel resistant then the Blue. Seems to stay stuck to the metal surfaces better then Blue so less problems with fuel and synthetic oil getting past it. Most autoparts stores stock it so easy to find.
http://www.permatex.com/products-2/product-categories/gasketing/gasket-makers/permatex-ultra-black-maximum-oil-resistance-rtv-silicone-gasket-maker-detailPermatex Red. I have not used it but after reading about it I will be giving it a try. Seems like a step above Ultra Black.
They also have this that looks interesting but no first hand experiance:
http://www.permatex.com/products-2/product-categories/gasketing/gasket-makers/permatex-sensor-safe-high-temp-rtv-silicone-gasket-detailThe Motoseal Gray doesn't say its Sensor safe. Id skip it for that reason alone on fuel injected motors (transmission Id consider it). It seems to be a product to be used in place of a gasket.
"The right stuff" Gray and black do say sensor safe.
They are stiff products that the excess will not come loose inside the crank case, bond well with metal and can completely replace gaskets. In fact some OEM use these products in place of Gaskets. Toyota uses this type product on oil pans from the factory. Its HELL to get off. That said after using a cork gasket on the oil pan (that had a RTV product originally) when I rebuilt the motor in my 4Runner...I wish I had used this instead. I think before its all over I'll be pulling the pan to reseal and I only have 20k on it now. :(
On a valve cover you need to get off periodically Id skip it for sure.