I use food grade mineral oil in a syringe. That way I can put a drop exactly where I want to. I like cutting apples and am tolerant of eating them, so I use food safe oils on my blade.
Lots of people recommend 10w nano oil, specialty greases (like the one that Chris Reeves supplies) or Benchmade's blue lube, and others will chime in with their favorites.
My personal experience is that the fit/finish of the knife, how polished the washers are (or bearing quality), pivot tightness, detent ball smoothness, lockbar tension, etc. all play a more significant role in the quality of the action than the type of lubricant used. The first thing you should look at is the pivot tightness. If there is horizontal blade play, tighten it. If it's too stiff, loosen it. If you can't get it adjusted the way you like, then you can start to look at the other parameters.
If everything else is dialed in, I guess the type of oil could matter...
My advice is to put less lube on your knife than you think you need. The more lubricant you put on, the more easily dust and gunk get in the mechanism, thus ruining the benefit of having the lube in the pivot in the first place. I use one drop on either side of the washer/bearing and one drop in the detent hole and I'm good to go. Even that may be too much, sometimes I'll wipe the excess of the 3 drops off the blade before putting it back together.
If you don't want to disassemble the knife, wash it out in soap and water, spray with compressed air, and put a drop of your lube of choice in the pivot. That would probably be more maintenance than 90% of the population does with their knives.