Are belt conditioners safe?

As a fix for a squealing belt... no, I don't like it. Find the root cause and fix it. As a tool for increasing belt longevity... they last a long time on their own, I don't think I'd extend the normal maintenance interval on belts because I sprayed that stuff on them. So, it seems like a waste.

Belt conditioning products are effectively Armor All spray for your belts. There may be some validity to claims that a belt conditioning product can extend the life of a belt by preventing the compound from drying out and breaking down. This is conceptually similar to how Armor All or comparable products help prevent old tires from cracking.

That being said, a belt cracking and breaking is not the only issue to be avoided. An old belt can stretch and wear down (just like tire tread) such that it is no longer holding your pulleys tight (even with an automatic tensioner). Moreover, you are spraying lubricating liquids on parts that function through friction, meaning you could cause a belt to slip on the pulleys which would effectively be the same as a belt snapping and not pulling the pulleys at all.

Spend the $7 (plus labor) to replace your belt. It is a relatively inexpensive component of regular maintenance and can prevent substantial engine damage in the future.

According to manufacturer Dayco (Why Belt Dressing Is Not a Solution To Quiet a Noisy Belt), it should not be used.

In the past, simply covering a noisy serpentine belt in belt dressing would quiet that pesky belt. That’s when belts were made of neoprene. Today, you should never put belt dressing on an EPDM-made serpentine belt. Belt dressing, and other oils and solvents, may quieten the belt noise when first applied, but these oils will actually cause the belt’s rib surface to become more aggressive once they dry out, leading to even more belt noise issues.

The article goes on to explain how it can actually make things worse.

the belts for my 93 Buick were 52 $ each plus the 1.5 hr labor. I had used belt conditioner often since my car sits 6 months then used again so I had concern about drying out in heat and no motion. My helo repair friend does not recommend it either. Maybe there is some "minimum dose" to try to help it but. . . . maybe aerospace 303 . I have used 303 on tires and other similar items and it really works great . ..I will give the belts a "dose" in some time. . . .