Anything close to motor oil will be fine in the Vulcan.
A durable and trouble-free design, although it has no power and is made of rude cast iron.
Most Aerostars were built with this engine as was ours.
After roughly 165K, it uses little enough oil that additions are never needed between changes, fuel economy remains pretty good, and it still starts instantly.
To answer your qustion, ours was back-speced for 5W-20, and I have run that oil once.
I mainly used M1 0W-30 for the first 40-50K of the van's life, and have since used a variety of oils, including RTS 5W-40, QS and Syntec 5W-50s, and various dinos including Tection 15W-40 (in warm weather months only), a mix of GTX 5W-20 and 20W-50 (to use up the 20W-50) and the current fill of Havoline DS 10W-40.
You can use any decent oil of any reasonable weight for the climate in your engine, change it every 5K, and the engine should easily outlive the rest of the truck.
