I used the K&N foam filter cleaner and oil for years on my MX air filters.
Considering I clean the filter every single weekend and never had any problems, I would say it's safe to use!
Now, I am using PJ1. Simply because it's a better deal (get a bigger can of filter oil which always runs out before you run out of cleaner). Can find it at crappy tire as well.
I use the filter cleaner stuff for other things as well, it's great for getting the oil off stuff!