I know that Mann filters are the best ones to use, especially considering the price of 3.50 per filter.

But many people praise the anti-drain back valve. I thought most filters all have an anti-drain back valve. When I removed cheapo Frams filters off the block, there's still plenty of oil trapped inside the filter. So is there really a difference between Fram anti-drain back vs Mann anti-drain back valve?

Though there's a big difference between the Fram filter element and Mann filter element. The Fram has considerably less folds and inferior quality than Mann.
Mann filters are packed tight with filter element.

Regards,