The main bearings of a bicycle are actually rather demanding, or so I was informed several years ago, by John Southey: a keen CTC member who also happened to be an applications engineer with Mobil Lubricants. He'd applied the techniques of his profession to the analysis of his hobby, and found that although bicycle bearings are subject to only moderate forces, these are concentrated on very small areas, due to the low conformity (dis-similarity in curvature) between balls, cups and cones in traditional bearings, or the very small balls to be found in any cartridge bearing that can be fitted in the same space. To understand what follows you need to realise that ball bearings do not actually have point contact, but compress slightly, as does the surface of the race, to make contact over a small elliptical area of shape and size dictated by their elastic properties and relative curvatures.
A lubricant acts like a cushion, increasing the effective size of the contact area so as to keep contact stress below a level that fatigues the underlying metal (leading to the pitting we have all probably seen on a hub cone etc.). Oil makes a particularly good cushion thanks to some special viscous properties of oil, in which its resistance to being squeezed out of a small gap increases dramatically when you squeeze it faster and harder. More viscous oils do this all the better, but the extreme pressure performance of a relatively thin oil can be boosted by "EP" additives. It's important to understand at this point, that grease is oil, mixed with a binder, usually some sort of soap. The binder acts like a sponge, releasing oil to do its lubricating job when squeezed, then mopping it up again - before it has time to escape. The binder also gives the grease its sealing properties and determines resistance to water etc. But the oil does the actual lubricating.
Bicycle bearings also rotate very slowly, which gives lubricants more time to escape from the path of the rolling ball, so this makes the lubrication problem all the more taxing. On the plus side, low speed means there's no chance of the grease overheating. This is a significant problem however, for the majority of other applications, which means that common greases that you'd buy for the car etc. will be less viscous than you really want for your bike bearings. And since most other machines have proper seals, these greases probably won't be all that good at keeping the water out either.
So what you want on a bike is a grease that has a high viscosity base oil (and/or a good dose of EP additives) in a binder with excellent sealing and water resisting properties that is also highly resistant to channelling (getting pushed aside and not flowing back). The one John Southey came up with from the range offered by his company was Mobilgrease SHC-PM. This is something they originally tailored to the requirements of the heavily loaded, slowly rotating rollers at the wet end of a paper mill. Slow, heavy, wet and dirty - sounds like a bicycle to me! Anyway: it's really good stuff, with the added bonus of being nice to handle, in that it gets its performance from a high viscosity synthetic base oil rather than EP additives (which smell like tomcats by the way!). I bought enough of that to last me a few years. But Mobil's product range
has not stood still, and from the several similar greases now offered I'd probably choose Mobilith SHC 1500.
Some of the greases that are (expensively) repackaged for the cycling market may well be similar stuff - or maybe not. The suppliers just don't provide the sort of data that might enable me to really tell. So I'm sticking with the industrial stuff, where I know what I'm getting.