10w30 is way to thick, gonna be a hot mess, need to back off to 5w20 at least, thin out the mix some...lol...any cooking oil will do fine.
You have to be careful with high heat in the FB with the highlander, you can (because I did) burn off the paint on the outside of the FB. But in my case it was because I had fire right up against the FB wall under the CC. Got way to hot and burnt off the paint, quick shot of high temp paint after it cooled off, fixed it right up, but have learned to keep the fire away from the FB walls by a few inches since, haven't had any problems after that...I also coated the outside of the smoker too, then wiped it down when cooled. I believe because I did that, I have never had any rust problems, and have had mine now for over 2 and 1/2 years, not one speck of rust anywhere.