Some of the the new machining processes used for aluminum surfaces create such a smooth surface that the OEM manufactures created new sealers for them. It also helps seal with the different expansion rates of the metals and even plastics in some applications. Previously, gaskets were often used but now just the sealant is often all that is required. Toyota, and some of the others Manufacturers offer a (similar) branded sealant. Toyota's product is gray and has some sort (?) of material mixed in it to give it some body unlike regular Silicone. It is somewhat like a paste and is sold in a tube. It is used most commonly by (Toyota) between their accurately machined aluminum surfaces in transmissions and some engine applications.
I was converted to a product call "The Right Stuff", made by Permatex. It comes in an aerosol can with a nosel on the top, much like a small shaving cream can. It is black and also has some body to it much like a semi vulcanized rubber, it does not squeeze out like silicone. It's a little pricey but the end result ustifies the price in my opinion. I use it for
oil pan gaskets (with the gasket), front cover, intake manifold joints also cast iron transmissions and everthing else on my engine at this time. It has essentially replaced Silicone for me, and the best part is there are " No Leaks."
Here is a direct link and description
http://www.permatex.com/products/pro...&item_no=33694
Rick