Most likely it's Cosmoline (rust preventative) applied during manufacturing. Most of it gets washed off at VPC and ends up inside the door where it seeps out - more so during summer months when it gets hot.
I used 3M adhesive remover ages ago or a mild detergent and hot water steamer to cleaner - it melts at a certain degree and wipes off. This time around I had it ceramic coated and the guys prepping cleaned it off.
Just don't let it sit for too long as it'll harden and becomes a pain in the arse to clean it off later.
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