Same as above from, cal22, There is no way I would use any kind of substance in a road wheel...If it stays wet it can settle, if it dries it can break away upon degrading or delaminating and you WONT see it happening!!!...but worse still is this scenario.....
You are going at say 60mph on an 18" rim...centrifugal forces act upon the substance you have inside the tyre, a small piece shifting inside the tyre will throw the bike all over the road, and now imagine that you have it cranked over on a long curve when THAT happens, centre divider crossed and suddenly going head to head with a truck dont cut it for me!!!
For a good reference point about the full effects of Centrepetal and Centrifugal forces see this info here, its the basic version..... http://www.freestudy.co.uk/dynamics/centripetal%20force.pdf
Best advice I am going to suggest.....I take my tyres off as soon as I notice any kind of wear, even or uneven, I RIDE mine and want to know that they are as perfect as I can keep them, just remember, if it has any kind of damage then it has the potential to kill you ... my regime is .....Tyre off, get new tyre ready, wash inside of new tyre, wash rim of both tyre and wheel with a very mild detergent that is thoroughly cleaned off, if its a tubed tyre, tube gets replaced wether it needs it or not, make sure inner edges of rim where tyre is seated is absolutely clean and clear of any dust dirt or debris, If you are not changing the tyre yourself make sure the person who is does it with clean gloves on-too many times I have seen people changing tyres without doing ANY of the above safety checks.....Would YOU Trust Someone Who Aint Going To Be Riding Your Bike To Do It Exactly To Your Standards?
The only times I have seen tyres fail is through negligence when fitting, poor maintenance checks during the tyres lifetime, and the obvious damage caused by some piece of roadside debris that punctures your tyre, its RARE to actualy have an unrecoverable blow out on a road tyre...
You are trusting your life to your tyres...If the tyre pressures are correct you have less than 2" of rubber on the road, would you want something inside the tyre that could lift that 2" off the road?
Heres a proof test of what I am saying.....get a small amount of duc-tape and make a pad approx 1oz in weight and tape it to your tyre, then spin the wheel by hand and see how it jerks around...Do you really want something you cant monitor inside your tyres?
I am sorry if it seems I go on a bit about safety issues, but I have now been riding on my 43rd year without ever having dropped a bike for any reason, have never had even so much as a gravel rash on a hand, have NEVER been involved in any kind of incident where even a tiny percentage of the blame could be mine, and, because I check everything on a very regular basis I know that I can still go out there and RIDE IT LIKE I STOLE IT!!!!!