I'm referrring to 'conventional' damper rod forks here
With stock springs (or worn springs) 20 wt oil is generally a bad idea although many here will recommend it
Thicker oil will increase compression and rebound damping.
Forks don't work properly if you hit a bump that accelerates slider on fork tube faster than oil can move through compression holes (fork 'locks up')
If you hit a series of undulations (small 'rounded bumps') with thick oil, the forks will 'pack down' which can upset handling pretty dramatically.
Under heavy braking, the forks hydraulic lock because oil can't move through the damper rod holes fast enough to allow suspension to work (which is why vintage racers like it, no front end 'dive')
Slightly thicker oil changes the range where forks operate correctly, it's going to make them work slower and lock up earlier if you hit a sharp edged bump.
Personally, I use 5w/20 full synthetic engine oil, it's smoother than 'ordinary' oil and doesn't thin out as suspension operated and gets hot.
The viscosity doesn't directly translate to ATF or fork oil but is probably similar to 7~10wt (it's thicker than 5wt fork oil)
Rocan is right saying to weld up damper rod holes, but, you have to drill new ones either same diameter in a different place (lower/higher on damper rod) or drill smaller holes.
The important thing is get spring rate right before you mess with damping