I use Team Associated Shock Oil in every one of my 30+ R/C cars. It's very consistent and doesn't thicken up in colder weather. It comes in weights from 5 Wt to 100 Wt in increments of 5. It's made from pure silicone, and takes a pretty long time to break down enough to warrant a rebuild in my bashers. I prefer using weights between 20 and 45. I wouldn't use anything higher than 45, and nothing lower than 20 unless it was a really different surface that I was running on. Touring cars react better to thicker oil, so if you have the cash it's a good idea to pickup a bottle of 40, 45, 50, and maybe 55. It's all in personal preferance. To me, that's a bit thick, but you're running touring cars on uneven spaces so you're gonna want something a bit thicker. Different manufacturers go by a different scale when it comes to thickness, so don't buy 35 Wt Associated oil and then replace it with 35 Wt Trinity oil and expect it to be spot on equal, because it ain't gonna happen.
If you're using Tamiya CVA oil dampers then you may look into getting a shock rebuild kit and seeing if it's possible to fit an extra O-Ring inside the shock body. Those shocks use a bladdered cap design with a lower seal system that's housed inside the main body of the shock, not in a lower cartridge like Losi, Associated, and Traxxas (Aluminum only) shocks. Using some Team Associated Green Slime on all rubber O-Rings will prolong the life of the seals and prevent premature leaking. It is a rubber preservation "Slime" that you rub into O-Rings to keep them lubricated and healthy. Dry O-Rings create friction, stiction, and wear. If your shocks were brand new it's possible that you forgot a spacer in between the dual lower O-Rings when you assembled them, or that There is a nice big divot in the upper portion of the shock shaft from a pair of misplaced pliers. When you go to replace the oil in the shock make sure to check out all the components inside the shock, as well as in the lower set of seals.
Good luck!