Don't do that. Thicker oil will just affect the rebound, dampening etc.
I know people have done this with ATF, but why? They sell other weights of fork oil... Just do it right for christ sake!
All you need to do is put a 1" long pvc spacer on-top of the spring in each fork, then reinstall the cap. You'll be very happy with the result.
If that for some reason isn't enough, time to get BBR HD fork springs or inverted forks.