This is exactly the plan in my head.

 

You might want to "Scavenge" the old oil to make sure you get it all out.

You can buy the kit from Rogue Choppers. https://roguechopper.com/

Here's a youtube clip showing how to scavenge ALL the old dirty oil:

 

Also M1 V-Twin can usually be had at Walmart for less than 12$ QT.

 

Are sure you removed the correct plug for the oil tank? Common problem, people pull the wrong one.

 

Not sure if you got your answer since I did not read all post. When a Softail sits, some oil since the bag is above engine will leak by the o-ring between the pump and the lobes on the oil pump gerotors that push the oil. It can also leak thru the one way valve flap on the filter. It ends up over on the pump and cam side. If it filled up to the crank center line, it could run over onto the flywheel side.
Mine will drop about 1/4" on the dip stick in the 4 months my bike now sits.
If I could not see oil in bag (use a flashlight), I would have pulled the plugs and with the rear wheel off the ground, spun engine with the rear wheel. That is the only way you truly know engine is turning over and is the way you manually turn a Harley motor.
If it was free, I would have cranked it with the starter a couple minutes. Since you say it was stored 4 years, probably would have shot a couple squirts of oil in plug holes. That should pump the oil back in the bag.
However, I sure would not fret it. It's fine. You changed the oil and filter. Ride it.

 

Hey Cencal, Your Dad's theory is nuts! It's obvious the oil in the tank got by the check valve, it sumped, to the bottom of the motor's sump area! So, the tank is full of fresh oil, & the motor is full of 4 year old oil, correct ? I would take the return line off the tank, (you need a little plastic release tool, $15. at the Stealership, $2. at Napa) the return line is the one on the left, the other is the vent line ! Put a clear 2'x3/8 hose over the return line nipple, put other end in a clear paint bucket, with oz. markings! Before starting, I would pull the plugs, fog the cylinders or a table spoon of mystery oil overnight, you don't want to break a ring? Be patient ! I'd put a new filter on, to trap any **** in oil tank ? Now, that your all set, start it up, pump 3 qts. of dirty oil in the paint bucket, shut it off! Technically, you pumped out the 3 qts. of dirty missing oil, & you should be left with 3 qts of fresh oil.but the oil pump immediately started pulling oil from the tank, pushing it thru the motor, dumping it on the dirty oil, this is not a perfect system, there will be some cross contamination, but, you will be back to a Full Tank, without a sump full of DIRTY, ACIDIC OIL! This whole sumping, flushing out should have been done with a cheap V-Twin Dino oil. If the oil light comes on, change the oil filter, don't take any chances! That's not the BEST motor H=D ever made, 1st year of the Cast Crank ! I have a tool that allows you to completely change your oil, I'm waiting for a Forum Advertising Sales Rep to get back to me ? I will have an ad in Thunderpress next month! Good Luck, & Merry Xmas, Boston Jim

 

Hey CC, You're right, you don't want to ever pull that pipe plug, it's not a drain plug, it's a casting plug, like a Freeze Plug in a car! And, it's in there with Super Duper Red Locktite type sealer! Even if you did get it out, it would drain the sump, BUT, you would lose your oil pressure, NOT Good! BJ

 

SO i think that I got it.

I ended up draining the oil in the tank this morning and starting the motor and let it idle for 1 minute.
Shut it down.
Checked oil level in tank and didn't register.
Started it again for one minute.
Checked level and it showed on the stick.
Started again and let idle for one minute.
Drained tank.
Refilled with the new oil that i drained earlier that morning.
Ran and idled for 2 minutes.
Drained tank.
Refilled with more new oil.
Ran for about 4 minutes.
Drained oil. Oil looked a lot better coming out this time.
Changed filter.
Refilled with new Mobil 1 Vtwin 20-50
Calling it good.

I'll run it 500 miles and then drain and replace oil and filter again and then it should be good for a while I think.

Now to find why the speedo doesn't work.

 

Try cleaning the speedo sensor....if no luck, swap it out for a new one.
Parts # 74430-00B - Sensor
# 11289A - "O" ring
# 3594 - Screw

 

Ok you are saying that if he drains his motor by pulling the crank case tapered plug that his is going to loose his oil pressure with the motor not running. So do you lose compression when you change your spark plugs