Hi Dave Greetings & warm welcome to the forum. Re: how much oil to use in a D14 ?, there is an oil cup on top of the steam chest, unscrew the cap & top it off, make sure to put cap back on as this is a steam escape route if cap not screwed back on. fit. a drop of oil on all moving joints is sufficient. no oil is to ever be put inside the boiler.
Will it cause any harm or damage if oil is put into the boiler?
Damage, no probably not, but it will cause your boiler to froth and spew in a manner that is not conducive to getting your engine to run well, thus it is a rather bad idea. It is also very hard to get the oil film that coats the inside of your boiler back out, so that it may affect its operation for quite some time to come until you do get it all out.
Another point to consider is that at the end of a run, it is generally best to open up your boiler (open the whistle) before it starts cooling down, so that as it cools it doesn't create a vacuum that sucks the oil out of your engine back into the boiler via the steam line!
Oops, looks like Charlie beat me to the punch.
As usual, you were much more articulate than me.