Yea, oil would probably not cause a bacteria problem, but you don’t know how the powder will react with it. The tadalafil powder is not soluble in water, or alcohol it seems, so it makes a pretty good suspension. It may be soluble in the oil, which could give you a nice clear solution, or it could turn it into a sticky paste that you can’t get to mix.
I have some vardenafil liquid that is a clear solution and seems to be more oily than the liquid C. The point is that V,L, and C behave differently in different liquids. The recipes I have seen for liquid V are complicated, and you don’t see liquid V for sale often. This leads me to believe it is not so simple as picking a liquid and dumping your powder in.