I'm really into BHO and mess arround with it's extraction for about 1 1/2 years now.
I always do a vacuum purge. With that you get most of the solvent out of it. Here is an example result of a lab testet vac purged bho, which was not made be me. We can see that there is residual solvent left, but in really small ammounts way below the "acceptable limit". It was made with Vector lighter refill gas.
Lighter refill gas isn't made from pure butane mostly. One thing one should care about is to use only brands, which have the "near zero impurities" logo. If that's the case one at least can be sure, that there aren't any anti rust agents in it or something like this.
Here is a list of some of the companies who are permitted to carry the "Near zero impurities" benchmark seal & are tested by Newport:
Newport
London
Colibri
Colton
Cora
CTC
Dunhill
Davidoff
David Ross
IMCO
K2
Unilight
Silver Match
Win
Sarome
And even those gases aren't pure butane. They mostly will have also propane in them. One can look at theis MSDS sheets to see what's relly in there.
Here's an example of an MSDS for the Colobri brand.
I personally use Unilight.
Everything is always okay in the end, if it's not, then it's not the end.