When I fixed my slipping treadmill belt on my Proform 995 SEL treadmill, I actually first tried greasing the track with treadmill silicon/grease as well, thinking that the track belt was sticking and causing the problem. I read later that you're actually not supposed to apply treadmill grease to the Proform 995 SEL treadmill. It also didn't help the slipping problem, but it also didn't make the slipping worse after I tightened the motor belt and fixed the slipping problem on my treadmill.
I suspect, that something is still wrong with your treadmill's motor and/or motor belt. If you think you tightened the motor belt too much, then try loosening it a tad and see if that helps. If the slipping gets worse, then I would try tightening it really good. I kinda doubt you will break the motor belt, but if you do, you should be able to get a replacement belt for around $20.
You may also want to try and clean the silicon grease off from underneath the treadmill belt using an old rag, and see if that makes any difference.
Source(s):
For those who haven't read my how-to post talking about fixing a slipping treadmill belt, check out:
https://www.gotknowhow.com/articles/how-to-stop-tre...