As for the oil pan gaskets, I had some heavy (yellow) wheel bearing grease left over from a farm trailer overhaul and used that as my adhesive as it was very tacky. I then used a red lithium grease to the other gasket surfaces, and this has served well for multiple crankcase & valve cover gasket removals.
From what I have read here & there, synthetic oils only really benefit new engines with tight tolerances; otherwise, for the larger tolerances in these flathead engines, any store brand dinosaur oil will have more effective detergents in it than the engine oils that were available 50 years ago. A good filter, sock or cartridge, will also assist the oil in removing contaminant from the system. The minor modifications needed to assist in sludge prevention in the form of the 180 or 195 thermostat and the PCV system can also help keep the innerds clean in the long run