This may be a dumb question, if so, sorry.
I understand that for most bolts on a car, they should be torqued dry (ie, no lubricant).
When watching some youtube videos on engine building, the bolts (big end in this case) appeared to be well oiled.
Neither the manufacturer or Haynes manuals seem to specify whether the bolts should be dry or not.
So, when tightening con-rod and main bearing caps, should one take care to dry the threads?
Similary with other engine bolts like the cylinder head bolts?

Unfortunately you really need to know the correct criteria if you wish to end up with the correct result. Often manuals contain a small paragraph somewhere at the very beginning with a statement along lines of "all fastener should be lightly oiled prior to torqueing unless specified otherwise" obviously that is amade up example

The purpose of "torqueing" a bolt is try and achieve the correct amount of tension in the bolt, often somewhere between 50% and 70% of the yield value, "torqueing" is one of the least accurate methods of achieving this due to the huge difference friction in the joint plays, lubricant and surface condition have a huge effect on friction.

If you lubricate a joint which is designed to be made up dry it is very easy halve the friction, this in turn leads to the tension in the bolt being doubled when torqued, in many cases this may not cause any problems, in other cases the bolt will be taken past it's yield point and will be permanently stretched and thus weaker, if you are very unlucky the bolt will break.

For standard materials e.g. 8.8 or 12.9 bolts and nuts you can use an online calculator which allows you to specify dry or lubricated and you will see change in resultant torque.

I haven't answered your question at all and I'm probably teaching you to suck eggs if you've done any research on your own.

Cheers
David

Thanks for the reply.
You are exactly right, I didn't read manufacturers manual quite carefully enough.
I looked at the pictures and the torque values.
The text actually says:
"Apply a light coat of engine oil on the threads and
under the heads of the cap bolts"
Thanks anyway.

If you’re using ARP bolts they come with a sachet of their own lubricant which they insist you use whilst torqueing up