In order to catch a ditto I have a fainted level 23 pokemon in first spot to use the repel trick in route 8. I also have some natus with synchronize abillity with different natures. However how do I know what Nature I need to catch ditto? Or any pokemon for thaat matter? Would any nature work? Does synchonize really help increasing the odds? By its description it only helps in getting the nature I desire, which is irrelevant so far to me.

Greetings Ariarticuno.
 

Repel trick is a way to find specific pokemons, which are the highest lvl spawning in an area. Using a repel trick usually means you are looking for a single, specific Pokemon. In your case, said Pokemon is Ditto. But the reason players use a sync for repel trick is purely to try and get a good Pokemon, that they might sell or use later: it has no impact on the spawnrate. If you are, as I suspect it, just trying to find a Ditto for the Bike Quest, the use of a sync is definitely not mandatory, as you will be giving away the Ditto to the NPC for the discount.
 

Ditto is also a very weird Pokemon, as it doesn't really need a particular nature to be playable, but wants the highest HP IV possible: it will indeed copy all of the stats when transforming except for the HP, making the stat boosts from the nature useless.
 

I hope I could answer your question, do not hesitate to contact me if you have further requests or question.
Have a nice day,
Caelesta

Thank you very much! i guess all the natu capturing was unnecesary for this lol. Case solved!