I have just finished my first squad of stormtroopers, but I've found that a straight wash of Nuln Oil produced a shading layer that was a bit darker than I would've like, could anyone recommend if diluting the wash with water would help or would I need to mix it with a little Lahmian Medium to keep it the right consistency?

I use Lahmian medium and it works well.

I've used water in the past and it helped but wasn't a great solution (literally and figuratively). I bought but haven't tried a medium.

There is another trick that could help, though ive never tried it myself it sure looks to work well.

if you give the model a coat of gloss varnish and let it dry then apply the wash with a thin brush into the recesses it will run along recesses and lines but without pooling or shading flat open areas.

i think its called "pinwash" and usually done on vehicles but it should work on any model

Sadly you have to pony up the cash for Lahian medium. It retains the consistency of the wash while thinning out the pigment.

Yes you have to use lahmium medium I'm afraid

Honestly any glaze medium will work.

Army Painter wash medium is a cheaper and just as good alternative to GW Lahmian medium.