i'm one for deffinatley sticking to manufacturers guidelines (least to use of purpose stuff not any particular brand) mainly as depends on marker/valve type as to what you use, for example my PE Etek4 needs oil on the bolt and rammer(being a poppit) and paintball marker grease for the low and high pressure regs.
As a general rule of thumb, grease is used for metal on metal contact, oil for anything else. But as above, followed the manufacturers recommendations. I use Planet Eclipse grease and oil.
Avoid obscure solutions
Note that some have began from need in the early days of paintball or for people living in remote places who could not buy from anywhere
Some you alternates just to save money. This is better served by using the right alternate, EG any Dow 33 grease as opposed to a manufacturer brand
Wrong types of lubrication will affect o-rings and may affect the gun
Either way you won't get too much help from tech support if your broken gun smells of chips
If you need to ask which to use then use what it says in the manual