The whole fishing reel oil debate reminds me so much of my former lives as a bicycle mechanic and a downhill longboarder and the staggering selection of lubricants on the market.
Obviously, there’s some differences in applications, but I think all fundamentals of geartrain lubricants are similar.
Heavy lubes will stick better to gears, etc, and generally are good for overall water repellency. The downside though, especially in a bike chain application, is that while it’ll stay on, it’ll also pick up enough grime and grit to make a street sweeper jealous, which overall will increase wear. I don’t know applicable the dirt–>wear issue would be for fishing reels though. The increased viscosity also may have noticeable effects on bearing freespin. Not a huge issue in longboard wheels, despite the notion that insane freespin = insane performance (I always opted for thicker stuff in my bearings in an attempt to keep water and grime out), but might show up in something like spinning reels if you like to give the handle a kick and let it spin itself out.
The way I look at it, in general, the more precise the operation, the finer the lubricant. On bikes with unsealed wheel hub bearings, we’d pack them with whatever can of grease was open in the back of the shop. Iron age technology = Iron age lubrication. I’ll avoid using chain lube analogies as the applications are totally different. However, on “precision” gears, such as in shifters, etc, I liked to use light greases of unknown composition. For super-precision applications, such as cartridge bearings, cable housings, derailleur linkages, I liked to use the thinnest teflon-based, non-wax bearing lubricant I could get my hands on.
Whatever you do, don’t use 3-in-1, I’m still not sure what it is other than a huge gunky mess that has no place outside industrial-type operation. And for the love of God, don’t use WD-40 on anything that you ever care about being remotely clean. The bike business taught me that WD-40 is like a hammer: it’s a repair tool, not something you’d use in regular maintenance. A classic example would be a squeaky door hinge. The hinge, under ideal circumstances, shouldn’t squeak, but WD-40 is decent for fixing it.