I'm going by 20 years worth of mechanic experience [email protected]
, and having worked on many different vehicles/ engines over those years.
It is possible that newer engine designs use different methods for sealing, but most of the vehicles I work on are of an older design. The original FIRE engine was first unleashed 21 years ago so some modern techniques don't apply!
I always follow manufacturers instructions, and also the instructions given by the gasket manufacturers. None of the head gaskets I have changed have ever required sealant or gasket cement, and all the instructions strongly warned against using them. Same applies with exhaust manifold gaskets, with the one exception being to use exhaust assembly paste as a last resort if the flange has been eroded/ damaged. This paste expands when heated and sets hard to seal an imperfect joint, unlike silicone which sets soft and doesn't have the same ability to withstand the high temperatures of the exhaust.
Inlet manifolds CAN use silicone type sealant, though normally it is Blue Hylomar or RTV sealant (Room Temperature Vulcanising). The inlet manifold doesn't reach anywhere near the same temperatures as an exhaust manifold so can use these types of sealant.
It's not a good idea to compare a modern Mercedes Diesel engine to an older Fiat petrol engine. Designs change, and what applies to one design might not apply to another. Also, diesels tend to run cooler than petrol engines (they don't ignite the fuel with a spark as they use compression instead - diesels don't use spark plugs except for glow plugs to aid starting!), so I'd suspect that their exhaust manifolds run cooler. That might be why Mercedes diesels can use sealant on diesel exhaust manifolds whereas older petrol engines must use proper exhaust gaskets.
If there is some new sealant that can be used as in the diesel Mercedes, can you tell the forum what exactly it is? What make? What are the specifications/ working temperature range? It would help a lot of people if we could get hold of some of this sealant!