string tension probably will not affect the spin on your shots (to any serious degree)....your swing plays 100x more of a factor in spin than anything else. What MIGHT effect the spin is the type of string you are using. Truthfully, probably not too big a deal though.
Forget about spin. Just play your game (changing it for different surfaces is dangerous)! The best way to get better on clay is to play on clay more. Movement is key on clay.
Although dissapointing, I hope the advice helps a little