I no longer use the wife's unit (I picked up a used Theater unit), but I always use coconut oil for the taste and texture. Not all that healthy :)
I do use Grapeseed oil for other frying when small amounts are needed (like the t-fal actifry).
A hint for you - get generic popcorn, and vacuum seal it into mason jars on a very humid day (or when it is raining). The kernels will absorb the moisture over a little time and pop up fluffier.
The kid and I usually get a 25# or 50# bag of bulk popcorn when it gets warm in the summer, and vacuum seal it into the mason jars for use over the year.
Grapeseed Oil from Costco has a high "burn point" so works very well with my Newman's Own popcorn. 2T. oil to 1/4 C. kernels. Just to be sure the oil is hot enough, I put the oil
in the Whrily and add three kernesl. When those pop, I add the rest. BTW, I love the Whirly Pop and over the years have given several as gifts. I think the popcorn made in them is SOOO
much better that the "microwave crap" so many people seem to like.