"Best" would be train specific lubes like Labelle. But use of others have helped my childhood trains run pretty good for over 50 years in my hands.
Roy Boy is seldom if ever wrong, but I haven't had pot metal issues and KNOW 3in1 was used until the late 70s. I just haven't seen or heard of oil caused deteriorations. That said both 3in1 and gun oils likely contain penetration chemicals that may affect some metalurgys, that I know, so I'd be wary to keep it off the porous body, etc.
I'd consider using the gun oil over 3in1, though both are kinda light, beats no oil. Gun oil (I use Stealth often, plastic safe) is great for wiping plate steel for rust protection and cleaning. Gramps compared motor cleaning to cleaning his Garrand. 3in1 is an old formula and will 'dry' to a varnish over time. You will likely be cleaning old traces of it as is. Some modern car 30w ish oil would be better. Lots of folks use Mobil 1 synthetic, platic safe, etc. More from the auto's shelves.. Lucas Red&tacky is a good grease used by many.
Kerosene may lift some paint...? (Test)
Naptha (Zippo fuid) should be milder, and is plastic safe, but still lifts some paints/inks. Test, & stay off lettering.
Careful use of Dawn and water (weak mix) brightens lettering best imo. (Done with Q-tips for care, Dawn goes after oils, ink and paint have oils, even dry) Dry dusting brush only on silver paint. Be wary of modern ink/paint as mfg. of near anything shifts to water based paints and inks.
Once apart, you can clean the body with soap and water, dry well, and quickly. Even steel washes well. The key is drying fully and fast, before corrosion can start..... and oiling steel to prevent rust forming slowly once dry. I've dunked motors and dried with a blow dryer fairly often. (That gun isn't trash if it rains, is it? 
) I dunk my window fans to clean them 3-4 time a season, letting all electricals dry for a day or so in the sun. I also put mother boards in dishwashers, etc. Etc.
Water ruins paper; mostly everything else just needs to be dried properly/promtly.
