Need advice on starting 88 P30 6.2 Diesel that has sat 8 years

I purchased a 1988 Cheverolet P30 step van with a 6.2 diesel. I am an amateur mechanic that has been trying to get it to start with the help of more experienced mechanics. We have been working on this for several weeks now. Here are all of the steps that we have taken so far:
1) replaced both batteries
2) drained diesel after putting diesel cleaner in the tank, then added 5 gallons fresh diesel
3) changed fuel filter
4) changed oil and filter
5) bled fuel lines until each line had diesel running out
6) checked glow plugs, they are working, relay is working, they cycle on and off
7) added oil to each cylinder
8) diesel is flowing through each line
9) manually turned the engine over with a wrench
10) fuel pump is working
I poured a little Red Marvel Oil in the cylinders when I first got it to loosen the cylinders up and lubricate them. I am not sure if that could have damaged something electrically. I opened up the air intake and poured a little in, about 4 ozs.

What else should we be checking? Is there an oil pressure sensor? I am thinking that it is something electrical. Some sensor that will not allow it to start. The engine turns over but has not started yet. We are going to give it a try again tomorrow at 11am. I am hoping that someone can share some advice who has experience with 1988 Chevy P30 diesel engine. Thank you in advance for any guidance.

I appreciate any advise. I have been reading through the Haynes Manual

Welcome aboard!

It sounds like you're off to a good beginning. The FIRST thing you need to do, before anything else is, remove all the glow plugs (test them while they're out, replace them if they're unmarked or AC9G plugs -apply 12V to the plugs, and they should glow bright within about 8 seconds). All that oil and MMO you poured into the cylinders and intake is great for lubing things up, but the engine won't care you didn't intend to fill it up with fuel (nearly any oil in a Diesel engine is fuel). If it did happen to fire, it would rev to the moon with no means to throttle it until all that oil burns off. Bad ending, usually. Before you continue attempting to start it, remove the pink wire connector from the top of the injection pump (Fuel Stop Solenoid) and spin the engine until no more mist/vapor blows from the glow plug holes. This will clear the cylinders of most of the excess fuel.

(Still with glow plugs out)
Make sure the batteries are fully charged (and keep them charged as necessary). Reconnect the pink wire (you should be able to feel/hear the solenoid cycling with key cycles -if it doesn't, check for 12V at the connector with key-ON, check IGN fuses). Loosen an injector line nut at a couple injectors. Crank the engine for 15-20 seconds at a time, at most, with at least 2 minutes between attempts (don't overheat the starter). Watch the loosened injector lines for weeping fuel. Once fuel appears, tighten the nut(s). You should also see fuel starting to mist from cylinders that don't have loosened line nuts. Continue until fuel is misting from most of the glow plug holes.

Replace 8 good glow plugs. Replace anything else you removed. Attempt a normal start. On a cold engine, the glow plugs should cycle for about 6-8 seconds (check that 12V is actually getting to the glow plug connectors). It should start, or act like it wants to. If it doesn't start, watch the tailpipe for white smoke. White smoke means it is getting fuel, but lacks heat to fire. It could be glow plugs, or low compression. Low compression can be caused by sitting for 8 years, but should improve with some running. No start and no smoke means no or too little fuel.

Keep the batteries charged, don't overheat the starter, and DO NOT use ether (starting fluid/spray, as tempting as it is).

Thank you for taking the time to give some advice. We will start by pulling and testing each glowplug. Then we will follow the additional steps advised of removing injection pump wire connection and blowing out excessive fuel. I am printing this out for tomorrow and will let you know how it goes.

Unfortunately my friend who is a more experienced mechanic who was supposed to help me yesterday was not able to come. I purchased a new oil pressure sensor and removed the glowplugs. I got new glowplugs to replace them as the old ones were rusted. I have a battery charger arriving tomorrow to recharge batteries and we are going to give it a try later this week. I will update this post after we take the recommended steps above and let you know how it goes. Thanks!