Fun? Yes. Permanent? Absolutely. Safe? No. Improper silicone injection can kill. If the silicone gets into your bloodstream it can travel to your lungs and do severe damage. Look up "Silicone embolism" for more information. Risk reduction techniques include: not getting more than 100cc at a time, and 100 is a lot if you're just doing your cock, at least 6 months between injections. During those 6 months do not pump or infuse. Have a professional do it. That last item is currently the most difficult. There used to be a popular and respected cosmetic surgeon "Dr. E." who was the name in North American silicone. However, he retired either during, or just after the pannacotta. The community is currently searching for a viable replacement.
In any event, I would recommend starting with pumping and saline infusion. The biggest risk with pumping is blisters. Saline infusions can cause bacterial infections, but following good IV practices reduces that risk to almost nil.
If you use Discord, send me a DM. I'll invite you to a server where silicone, and other genital enhancements, are actively discussed.