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I'd like to grease my seatpost, and in front of me I've got a tube of "CMD Extreme Pressure Lube #3" that claims to be good for "ball bearings, roller bearings, deep drawing, rivet spinning" and a host of other industrial applications I don't understand.
Is this likely to work well?
If not, what makes grease appropriate? I'm sure I could pick something "specifically intended" at the LBS tomorrow.
Phil Wood, the owner of the bicycle tool and components company of the same name, once commented that his number one requirement when he was buying barrels of grease for his company, was that it be the exact same green color shade as the previous batch, so that he didn't get too many phone calls asking what he'd changed.
Point is that the "bike specific" greases generally aren't.
As far as functional necessities:
Not too thin - A thin grease, like Slick Honey, will run very smooth, but only for a very short time. If you want ideal performance, and don't mind overhauling your bike every couple weeks, use this.
Not too sticky - Motorcycle/Automotive type bearing grease tends to be very sticky and very thick. They are designed to thin out at higher levels of heat and friction than a bicycle will reach. So it will last a long time, but will feel tight, and add friction to your bearings.
Generally, buy a tube of Phil Wood or Park tool grease, and feel free to use anything with a similar purity and consistency in the future.
Edit: After reading Matthew's answer on this page, I realized I hadn't pointed out the 2 cases where the grease matters. In the case of Carbon, see here. In the case of Titanium, with any non carbon frame, you must use a copper paste grease, commonly sold as TiPrep. it is messy, annoying, gets all over everything, but it stays put, which no other grease i am aware of does for Ti.
The best grease is the grease with the least amount of contaminants. Everything else is secondary to that.
Naturally grease does not come with contaminants in it, but, in the workshop it attracts any dirt going, as if it were some huge magnet.
Therefore, packaging is important. The large tub that you have for the car/motorbike/boat/unicorn cage might not be too well suited to the bike because the small quantities needed for re-packing your hubs might be hard to cleanly dispense. Hence the small tubes, as sold in bike shops, are worth the money even if you already have a vat of the stuff at home. With a tube you can always wipe the end.
There are affordable tubes of grease, e.g. the 'Weldtite' line of 'lithium' grease in the UK, these work absolutely fine and have plenty even in the smallest tubes. There are also more expensive grease tubes with fancy dispensers and fantastic formulations - imaginably these are vastly superior but, in practice, the grease is secondary to pre-load, part wear and correct assembly.
If you had two identical bikes, one packed with the posh grease and the other with the affordable stuff and did some 'blind riding' I doubt you could taste the difference or find that you would need to service one earlier than the other.
On the other hand, if you had a bike refurbished by an experienced mechanic with normal light lithium grease and an identical bike refurbished full of posh marine grease by someone that has to think twice about which way to tighten a bolt then I think the former would have noticeable ride quality.
Sometimes a purchase is of psychological value. I would prefer to have my Campagnolo parts with Campagnolo grease, my Shimano kit with the 'correct' Shimano grease and not feel 'let down' by cheap grease. The affordable lithium grease is light (which is correct) but one imagines it to wash away.
As for your application of the seatpost, the stuff you have will probably work fine, so long as your seatpost is not carbon fibre or your frame not be titanium. Otherwise, you must grease your seatpost unless you want to use the blowtorch to get it out again. The best grease for the seatpost is the light lithium stuff - a light coating of the affordable stuff wins because you don't have uneven distribution or it too greased (needing the seatpin bolt to be massively tight). A third benefit is that you can clean the excess a bit easier.
For greasing a seatpost you wouldn't use the same grease as for a bearing. For the seatpost you want a "clean" grease, and one that will stick well, not a grease designed to stand the extremes of wheel bearings. "White grease" or silicone grease would be options, or even a heavy oil. A hardware store will have options, if you describe the application. (Not that any great harm will occur from using the grease you have -- it's just that it will be unnecessarily messy and will wash away in the rain a bit too readily.)
For bearings you'd best use Phil Wood or Park grease from your local bike shop (or online), unless you have an exotic bike that specs something different (in which case follow the specs to the letter). A small tube of bike grease will last a good while, and it's not a major investment. But the stuff you have is certainly adequate, and I wouldn't hesitate to use it if it was the only thing at hand.
Grease generally serves two purposes in a bicycle:
Decades of experimenting and I still am a sucker for the 'next great thing.' A few tidbits I do trust, now:
Tiprep antiseize for every bike part that is tightly pressed/bolted infrequently, especially dissimilar metals where galvanic corrosion will occur; Ti's bad, but Alu to Alu or steel/Ti may be even worse. Greases are NOT antiseize, nor is it a lubricant, but a high pressure thin layer boundary.
Blue loctite for every other nut/bolt, including outside locking ring threads on cassettes, because I've seen several loosen over time. Also acts as a corrosion protection, but with benefit of controlled vibration-proofing.
Krytox full strength - pricey, but best for sliding friction metal to metal, like in shifter plates, cable lubing. Is pretty dry, low dirt collector; NOT a great bearing grease, as it seems to dry out or clump, w/o cling.
Pedals- Shimano grease lasted for years, pretty clean, but my high tech Viperlube seems to be either washed or spun out in a couple hundred miles.
None of the common greases have proved as durable (nor very water resistant) as the original when bearings get re-packed, a mystery as maybe Shimano stuff really is good. If you're handy, try each side of a hub/crank with different stuff, and record over time which looks cleaner/newer with each service.
Most auto lubes focus on extremes of temperature, corrosion, and pressure, and either get pumped/circulated in closed areas, or held in boots, so are too thick or too thin or too sticky for bikes in most spots.
