WD40 is a penetrating oil designed for metal applications. I would not use it on rubber products.
I doubt if there is much change in the belt lifetime associated with any lubricant. Let's hear what others say.
a friend of mine who ran a Shell station in Baton Rouge once told me to use LAVA soap, on the inside surface of course, while its running(!!!); this is for squealing belts that seem to be tight enough; cheap-o 'belt dressing'......(it worked, but you better be careful or you'll lose a finger or worse!)
My dad was a car racer back in the day and he used to say all the time that the best thing for the belts is: when the car is off, spray or apply coca-cola all over the belt and let it dry before you start the car. i guess the sugar in the coke makes the belt not to make those anoying noises.
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