Loctite, or thread locker because I typically use the brand Permatex, is for helping keep a bolt or nut exactly where you want it until you want/need to remove it with a little extra force. I am well aquainted with doing this because I ride a Sportster and they have a tendency to rattle off any and every nut and bolt they have.
On the other hand I don't use grease to do the opposite, I use Anti Seize. Loctite makes a version of this product as well, but again I use the Permatex brand. Anti Seize makes it so you can easily remove a bolt that has been in place for long periods of time, in an environment where rust is unavoidable, or in places of high heat which can cause metals to bond/expand/retract and all other manner of annoying things that will make the bolt or nut difficult to remove without a little lubrication.
Grease is really more for an area or mechanism that moves such as joints or bearings. It can also be used to slide parts into place but again there are other products such as WD40 I would rather use than grease. I also sometimes use grease to hold a gasket in place if there aren't guide notches to do that job.
Loctite comes in 4 flavours. Green for electrical insulation then Purple, Blue and Red for weak to strong adhesion. Its applied to the threads of bolts before fitting the nut to hold them in place. Not as effective as lock-wiring of course.
Grease is used on moving parts to reduce friction and wear such as brake pistons, axles, chain etc. Or to seal parts from the elements and moisture.
The only time I would put grease on a bolt is on the outside once its tightened to protect from water and corrosion ingress.
WD40 is a DEGREASER and good for cleaning but NOT for lubrication as it evaporates and dries in a short time.
GT85 is better as a low viscosity lubricant but is also easily washed away and not suitable for long term lubrication.
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