Honestly I would have said if you're happy with Nuln Oil, just stick with it
The Morkanaut is a (googles) $105USD model

A bottle of Nuln oil is $7.50, at most you're going to use, what, 1/4 of a pot if you slop it on decently thick? So that's $1.88 worth of wash on a $105 model, personally I'd be saying "screw it, just use the Nuln Oil".
If you want a cheaper option, you can buy an ink rather than a wash, they're much higher density and you have to thin them down if you want to use them like a wash. So a $7.50 bottle of Nuln is 24ml, a 17ml pot of Vallejo Black Ink is about $3USD (or use Daler Rowney inks, or P3 inks, or Liquitex inks), to use it as a wash you thin it down probably about 5 to 10 times, so you end up with 85 to 170ml of wash from a single 17ml bottle of ink.
Lester Bursley famously has a wash recipe...
https://www.dakkadakka.com/dakkaforum/posts/list/261541.page
Personally I'm too disorganised to do that, so I just put a drop of ink on my palette, 3 or so of drops of vallejo flow improver a maybe 3-5 drops of water and that seems to work okay.