10W-30 or 10W-40 for whatever climate you're in. Not sure about the additives though. I don't think they're needed though. I'd only use other oils if...
The wire that goes to the small peg on the starter solenoid. Trace check if 12V is getting to that connector when switching to start (creative means...
I'm running an even larger one (24 plate) on my 93. Towing 3500# at 70mph, return temps only ever got up to 160F or so. This is cooled fluid, so it's...