Hi there,
I have always used chassis grease to lube both the threads and the shoulders of the head bolts. This allows for very consistent head torque measurment. In over 35 years and over a dozen engines or more I've never had a problem. It's especially important to use some kind of grease or anti-seize when the bolt is going into threaded aluminum. If you don't it might not come back apart again.
By the way, here's a tip for removing bolts that have broken off in a head or block - use a mig welder to build up the broken piece to a height that you can weld another bolt on the built up piece. Then very carefully back out the bolt. The welding causes enough heat so the bolt or stud usually comes right out with little effort.
Another tip is to ALWAYS chase the threads in a block using a tap, before installing the head. This will help ensure a consistant torque, and if the hole has oil or antifreeze in it, it will help get rid of it.
Here's another tip, and an important one. If you use a silicone sealant, like RTV, etc., be sure not to use too much. I've seen some very shoddy jobs where engine passageways were actually plugged by silicone sealant. The way I prefer to do it is messy but works well. Apply a small amount on each side of the gasket surface, then work it in with your fingers, removing any excess material. It really doesn't take much at all to do a great job of sealing. Make SURE that the surfaces to be sealed are absolutely clean and DRY. The RTV will not seal if there's any oil or water on the surface to be sealed. Also, don't use it where it is exposed to gasoline - it will dissolve in short order. It is good for sealing oil and water, not gasoline.
Regards,
Richard Wooldridge
'82 300D/4.3L V6
Etc...