My friend's a mechanical engineer and he's helped me understand about greases, gears and all kinds of mechanical things over the years so here's my version of his 2 cents. His mantra is "any grease is better than no grease", so it's probably not so important what kind of grease you use for plain bearing applications and sliding elements like tractors use.
Seals which have been exposed to one type of grease can degrade if you're unlucky when you pick a different kind (not brand, a different type) of grease with conflicting chemistry. So don't switch greases once you pick one. Clean the fitting first and flush all the old grease out each time you grease. When grease oxidizes from heat, or becomes contaminated with moisture, rust or corrosives, it needs to be purged. Similarly if it exceeds it's drop point, the temperature at which the base melts, it does not return to it's original composition and needs to be removed. Alternate implement positions each time you grease, ie roll the bucket forward one time, backward the next.
Multipurpose greases are generally the best if you're positive that the lowest drop point of all the components is not below the service temperature. White lithium grease is supposed to be a very good grease with wide operating temperatures but I was only successful finding a data sheet on the product made by caterpiller, something that isn't available in stores. Without knowing the recommended temperature range it's kind of flying blind.
Special purpose greases are required for certain applications, like high pressure greases on gears and plain bearing applications on tractor loaders. These have lower temperature operating ranges, but the grease has to sustain much higher shear forces.
It feels like they're using the lubricant on me when I have to pay those high manufacturer prices, but they also paid someone to figure out which kind of grease to use in the first place so I just try to remind myself I'm creating jobs.
