How to fix a P0300 Code for a 2004 Chevrolet Express 2500 V8 6.0L?

What is your question?

This Van has been sitting for approximately 2 years. I put some Seafoam in the gas, checked the fluids(oil,brake,power steering) swapped the old battery for a new one, checked the air filter, then sprayed some starter fluid in the Carburetor and started it. After starting it I moved it into a better spot to be worked on. Then I proceeded to drain the semi chunky oil out, remove the old oil filter, and let all the oil drain off as much as possible. I then put a new filter, new oil, Marvel Mystery Oil in, then started it up and let it run for 20 minutes. And then changed the oil and oil filter again after that without the Marvel Mystery oil this time. But when I started it up the Check Engine light was flashing yellow. After clearing the light it came back on about 5 minutes later. I then saw it was throwing a P0300 code. I don’t have much mechanic knowledge. So considering that code means an engine misfire I guess I will change the Spark plugs next?

What makes this problem better or worse?

The OBD reader I used suggested replacing the MAS or cleaning the injectors.

How long has your 2004 Chevrolet Express 2500 had this problem?

A few days.