There are two things in this world I abhor, WD-40 and duct tape. Both have their place, but both are more often than not, the wrong tool for the job.
But about WD40... It is not a lubricant!!!!! It acts more as a solvent/cleaner in most instances.
I have worked in the maintenance field for over 20 years and in that time, I have seen it misused and abused in about every way possible. I have seen it cause more problems than it solves. It does have its uses, one of which IS NOT LUBRICATION. Whatever you want to lubricate with it, don't. There are better options.
What it does do well is:
SHORT term rust protection (days)
Cleans up sticky messes
Bug and tar remover
Displacing water from pneumatic systems
I work in injection molding, the stuff is awesome for cleaning up the dye used in Liquid Silicon Molding.
Now, a gun related story.
Years ago I bought a Phoenix Arms HP22A from a co-worker.
He carried it in a leg holster and frequently lubed it with WD40 since it got lots of moisture.
When I got the gun, he included the mag full of CCI Stingers.
So when I go to fire the gun, first round sounded kind of puny.
I check, nothing in the barrel but you could tell the round was weak.
So I try again, same thing.
The WD40 had soaked into the rounds and they would still fire, but were so weak they were basically falling out of the barrel.
Put fresh rounds in and everything functioned fine...err, as well as could be expected considering the POS Phoenix Arms pistol...but that is another story.
Anyway, in the case of the OP's question, WD40 would serve well to displace the moisture from your Mark I. I would then wipe it clean or blow it all out with an air nozzle then use a good gun oil like Break Free.