Your correct in that its normal. Just a additive that seeps out the side walls to protect against UV and cracking.
Elbow grease, stiff scrub brush with Simple Green, 409, etc... will remove it.
It will come back however.
I like 303 Aerospace Protectant, has a good UV shield, doesn't sling, more of a mat finish and no silicone or
oil that can harm the rubber like some of the glossy ones. Rule of thumb usually is if it feels like Baby
oil steer clear of it.