Last owner apparently not a big fan of changing the oil

You've had it running, I'm sure.
Warm it up and Here is what I'd do.
Step 1. Drain the oil but leave the filter on it with the oil that's in the filter.
Step 2. Pour 1 gallon of diesel fuel in it.
Step 3. Start it and let it idle for 20 seconds.
Step 4. Drain it. Let it drain till the drips quite. Put a new filter and oil in it. Change the oil/filter within 200 miles. Good to go :)