We really need to stop spreading the myth that thicker shock oil helps "stiffening" a car. It doesn't. It slows down the compression/extension, but pressure required to compress the spring is exactly the same. It slows down the compression by increasing pressure inside the shock - all you do is increase the potential to blow the top off the shock, or bend the shock shaft because the shock doesn't compress fast enough.
As a rule of thumb. Oil is used to tune how fast your car goes from full compression to full extension and vice versa. Not how much force full compression takes.
The only way to increase spring pressure "or stiffness" is by using stiffer springs. There's no other way. Everything else is wrong.
Now, you actually do want somewhat soft springs and reasonably light oil on a basher. Within reason. Otherwise they won't last (generally by either breaking the piston inside the shock, or bending the shock shaft).
There's a simple test. Adjust the ride height to where you want it (as was pointed out by wombat - directly under the shock cap is a collar that you can wind down, adjust that down to increase ride height to where you like it), take the rig and drop it from 3 feet. That tells you exactly what you need. If the chassis hits the floor, you need stiffer springs. If it wobbles or waves, you go up a little in shock oil weight - it should drop on the wheels, compress the shock all the way without slapping on the floor, then extend straight up without any oscillation to "resting position". If it goes up all the way to full extension and then settles back down in the "droop" from the weight, your oil is too light.
That's your general tuning starting point. That's how most RTRs behave at least, and i found it relatively easy to tune from there.