I noticed the owners manual says if doing a lot of towing to change rear axle fluid at 24K or something similar to that. I'm almost at 19K, and was wondering what you guys recommend. (mileage wise) I have the limited slip/antispin rear end 3:55. I have looked at both vavoline and mobil synthetic rear axle fluid and they say for limited slip. Just wondering what limited slip additive I need to add?