It just seems to me a that sac oil Is a none aggressive type of smell. While castor Is a aggressive type smell. It seems to me that the 2 shouldn't be mixed. I know It's done but I don't.
I just use mineral oil or glycerin mixed with my castor to make It more of a liquid.
Nothing wrong with not mixing them. I've used straight castor that I cut out of beaver while on the line before and did fine. I just grew up learning to mix them (and my dad said he tried mineral oil and didn't have as good of luck as with sac oil, so I never tried it, funny how at over 40 you still do things because that's the way dad did it). I've never had a beaver refusal problems with castor/sac oil mix except for trapwise beaver. I have found that castor sets are absolutely deadly when trapping rivers and swamps, and while they still work well when trapping ponds in small streams where the beaver don't travel as much they aren't as dynamite. Since such areas usually have more blind set opportunities than rivers/swamps I tend to mostly blind set them.
I always believed that a small amount of sac oil is secreted when a beaver makes a castor mound, so always thought it was more natural to have some mixed in.