Decaf is always a bit weird.... due to the processing, starting at a brown color, no chaff, blah blah blah.
In terms of oil vs. second crack...
Significant oil appearance really shouldn't be able to happen before the second crack as far as I know. Certainly some oil, but a decidedly oily appearance should be post second crack territory.
The heat gun may create some weird effects. I don't know what the 850 degrees "means", but that seems quite hot for any sort of ambient temperature in your roasting chamber, so it's hard to know what it was at, or more importantly, what temp the beans were at. Beans should 1st and 2nd crack at relatively specific internal temperatures. 350ish for first, and I believe 390ish for second, though I don't have a reference handy at the moment.
In any case, if you have some pictures of the beans, it'd give everyone a real nice picture of what's going on.
For certain coffees that have trouble getting to second crack, you might try backing off the heat for a little while and going back in. There'd be less risk of burning with a more gradual rise in temperature. (Especially when the heat source is so powerful)