I have used STP and/or Motor Honey in steering gear boxes for years, like fifty plus.. It does not leak out of the steering gears..
The STP/Motor Honey is a little difficult to put into the steering gears. I have found that the it works best to sit the STP in the sun for awhile, insert a small funnel into the box opening, slowly pore the STP in. pausing now and then if the funnel fills..
The gear box will only hold less than a half can/bottle..
I used STP in a 8CM engine in a boat I had in the mid '60's.. The engine was getting a little tired.. One weekend the engine blew up... No fun.. I was going to convert the boat to SBC.. A friend of mine had a F-5 truck that had a "new" Ford engine it. My friend gave me the truck for $100. I pulled the engine, it was an 8CM..
When I removed all of the marine equipment from the original boat engine I found that it had an over size pan. Removing the pan I found what I thought was the blatter from a football laying in the bottom of the pan.. Hmmmm? When I tried to pick the "blatter" up, much to my surprise I found a huge mass of STP.. So much for STP's claim that it would MIX WITH ALL ENGINE OILS..
At the time I was using Union Oils "Guard-All" 30W detergent oil.. I had switched from Union Oils Royal Triton, (Grape Juice) because UO quite making it...