Ok, i got it, so that "copper based gasket silicon sealant" made by loctite is pretty much what i was saying when i suggested to use the gray silicone to seal engines
The difference is the gray one is cheap and it comes in big tubes... and the one made by loctite have copper particles + his chemical formula, etc...
But at the end of the day both are silicones and are used for the same purpose... so i guess the flexibility is going to be pretty much the same
The good thing is you can "cut" them with a plastic (e.g. a credit card) if at some point in the future you need to remove it