I am about to fit the cylinder head to my Mk.3 with a copper head gasket supplied by Andover Norton. After annealing, they recommend a layer of Hylosil (Engineering grade RTV silicon) to both surfaces.
When I Google the question, I get a LOT of different opinions.
What is the current thinking on this?
It will vary....
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Also using a copper head…
Also using a copper head gasket, Jim Schmidt recommends "Pliobond as a sealant and to place .005" copper wire around the pushrod tubes and the oil drain holes crossing the ends.
This method is a variation on a theme by Anon who used fine cotton embedded in the sealant in a similar way works a treat.
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Head gaskets
The current 06-5051 composite gasket is perfect for my 850. Easy to fit (dry) and remove. No sealant, annealing or descaling required and leak proof. Re torque after a couple of hundred miles then forget about it until the next strip down.
Copper gaskets can seal well but have far too much prep work for no clear advantage in my case although I always anneal Aeroquip and other solid copper washers.
The Jim Schmidt method Paul mentions to eliminate leaks with copper gaskets is described here: https://m.youtube.com/watch?v=TUnQg2TNWHQ
The leaky copper head gasket fitted to my bike when I bought it ended up on a nail in the shed where it remains as a handy source for custom copper washers.
Most heat generated in an air cooled engine is around the combustion chamber and cylinder head hence the heavy finning compared to cylinders while heat is also carried around the engine via the oil.
A composite gasket will have a minimal negative effect on engine cooling. Heat from the cylinders will interact with the head (and crankcase) regardless of gasket type especially in heavy traffic or after a run. A lot of heat also exits via the exhaust.
My deeply finned Sir Eddie alloy rocker covers with standard AN gaskets get very hot in use and no doubt play a small but important part in cooling metal and oil.
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Wellseal sealant
I agree with Ian Soady about using Wellseal. I also use solid copper head gaskets on my 750.
Smear wellseal over both sides of the gasket and leave to go tacky for a few hours or even overnight.
Then just fit the head on with the gasket and tighten it up once the push rods are in place.
It's really good stuff and not complicated to use.
I'm afraid you will lots of different opinions of what to use and how to do it.
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... but I use a thin smear of wellseal which seems to do the trick.