short version: I refreshed an M42 (BMW) engine....head-off, new HG, cleaned up the cylinder bores, valves, etc. Did NOT rebuild the block or anything in it, nor clean up anything in there (so still has oil residue on the crank, etc). Engine last ran about a year ago.
Basically, I recall hearing that people like to put some marvel mystery oil into the cylinder through the spark plug holes before starting an engine in this condition.
Also I'll plan to crank it over a bit with no spark/fuel to get some oil circulated before the "real" start.
Anyhow, marvel mystery oil - old mechanics' tale, or legit?
any other suggestions to make sure no damage is done on the first start?
just start it like any other engine that has been sitting for more than a couple of weeks. no need to smoke out the entire neighborhood by putting oil in the cylinders, and no real need to crank it without spark unless it makes you feel all warm and fuzzy to do it.
I don't lube cylinder walls with more than a touch of WD40 on fresh rebuilds. I wouldn't do anything for something that was already together and running.
I've used it for engines with dry or rusty ring to free them up, so I'll offer a word of advice: When you get ready to hit the starter with the plugs out to blow the mystery oil out, make sure no one is standing over the plug holes (ask me how I know!). I'd throw an old towel or something over the motor, because the mess goes everywhere (ask me about the Mystery oil stains on my garage ceiling 
)
Did you hone and are there new rings?
If no, then as far as the cylinders are concerned, nothing changed. I wouldn't put anything, or at most a thin layer of motor oil.
If yes, I kinda like 2 stroke oil to lube the cylinder walls on fresh ring jobs.
I was introduced to Mystery Oil by my father who used it for everything. I still use it on occasion.
In your case, as I understand it, MO is not needed. Just start it.
Now if an engine has been left sitting undisturbed for a long time, then yes Mystery Oil is the answer.
Hey, it even fixes automatic transmissions.
I'm about to start an engine that hasn't run in about.... 4 years. MO?
I've always cranked w/ plugs out til it showed oil pressure, then lit it off and allowed to idle 'til warm.
First drive I'll do 1/2 throttle roll ons to 1/2 of redline times about 20, then some 3/4 throttle to 1/2 of redline, 3/4 to 3/4, full to 3/4, and finally the whole thing.
We used to break in absolutely fresh new race motorcycle engines on the dyno. Strap it down, make sure it's up to temp, a half dozen roll ons to seat the rings, and immediately start full efforts to rev limiter. They always looked great when they came apart for refreshing.
sounds good gents, thanks.
