I'm staining a butcher block countertop with pure tung oil diluted with citrus solvent after progressively sanding with 60 & 120 grit sandpaper. Is wood conditioner also needed before applying the first coat of diluted dung oil?
You're not staining so no, "wood conditioner" (terrible name for the product!) is not needed.
The purpose of "conditioners" is to partially seal the surface of woods that are prone to blotching, which is taking stain very unevenly. Pine is possibly the worst offender for this but a few other similar softwoods are prone to it to and it's not confined to softwoods, cherry is a notable example of a blotch-prone hardwood.
You didn't ask about these two things but I think I should cover them
Sanding to 120
Generally when you oil wood you want to be particularly careful about how well the surface is prepped and as part of this you usually sand to a higher grit than you would if you were using a film-building finish such as shellac or varnish.
It's rare anyway to sand only to 120 grit. 150 is about the lowest grit usually used for finish sanding, with 180-220 probably being the commonest final grits used.
When oiling it's not uncommon to go to 320 grit or finer and in fact you can sand very finely indeed without issue because there are no adhesion problems with an oil like there can be with a film finish.
Thinning oil for application
You don't need to thin oils used in finishing. And in fact I would recommend specifically that you don't for a couple of reasons. Primarily it's because thinning really has zero benefit. Yes a great many people do it and convince themselves it helps with penetration but anyone can do some side-by-side comparison tests for themselves and confirm it makes absolutely no difference at the end of the day.
When you oil wood the idea is to get as much oil as possible into the wood fibres and diluting the oil just ensures that each application puts on less oil. So in essence thinning oils merely lengthens the finishing process.... which is already tediously long-winded when you're oiling :-)
I know this post is old, but I wanted to mention for anyone that stumbles upon it, that for 100% pure tung oil, you DO want to thin it with citrus solvent. 100% pure tung oil doesn't penetrate well on it's own and is very different than most other oils that don't need to be thinned. Every single company that sells 100% pure tung oil (not tung oil varnish) and wood working forums recommend thinning it to 50/50 because it greatly increases it's ability to penetrate into the wood. So while it might take you longer maybe because you are using less oil, in the end it's far better because without the citrus solvent (or mineral spirits if food safe doesn't matter) you will get far less penetration into the wood.