There are a ton of fork oils out there. Compared to what was in mine, about 15 ml of goo, I suspect most of them should work well.
Do you guys have a preferred wt?

I'm curious to hear what others use. On my mountain bike I'm pretty fussy about using the correct oil according to the mfg, but on the vintage motorbikes, since the technology is pretty low and the manual calls for 30wt I just us whatever synthetic 10w30 is on sale at the time.

Also came back to edit that as far as fork oils go, consider the wt more a code that a rule; it's not standardized so it can vary quite a between between brands. For example I recently switched a fork from air to coil over, and coil manufacturer said "We've tested with Fox 20wt Gold, WPL ShockBoost 20wt, and Rockshox 0W30 (which is approximately equivalent to a 15wt at room temp) and all of those will work fairly well."

I used 20w fork oil and think it's a little harsh but not enough to change it.

10W30

My 74 DT360 I use straight 30W, found it at O'reilly's auto parts

My current inventory

Only time I went specific brand was with an Ohlins cartridge swap in my Yamaha FZ09. I had spent so much on the hardware I didn't want to take any chances on the performance of the fluid. So I installed what was recommended. There is some left over and that is annoying... I'm always fighting for space in my chemical cabinets.
For awhile I thought because the vintage Yamaha is so heavy the fork oil should also have a high viscosity . So I was using 20 W fork oil. Then I found PJ 1had 30 W and I tried that . It was fine for the desert which is slow terrain. Don't get confused here you can go very fast in slow terrain. Bumps generally are spaced far enough apart to allow the slow action of the 30 W to work.
Now go somewhere the ground has clay with a high amount of usage. You will find what is called "chop". The front end will float because the rebound is to slow with the 30 W. In technical sections or fast terrain ( chop ) 7W or 10W is way better at keeping your tire in contact with the riding surface.
So lately I have been mixing witches brew = Using left overs to get to around the 15W territory.
Evan the modern stuff gets stale so if its been a couple years you are due for some fresh.

One problem is there is no real standard for fork oils... one manufacturer's 5w can be thicker than another's 10w.

I like the Torco stuff because of the fairly high Viscosity Index which just means the oil thickness stays consistent over a wider temperature range.

The best resource for data on suspension oils I've found is Peter Verdone Designs....

www.peterverdone.com/suspension-fluids/






.

It's pretty subjective, but I use Maxima 10W, or Dextron ATF in a pinch for the DT, or most old bikes with damper rod forks.
I might bump up to 15W if I need to stiffen things up a bit, or lower viscosity if the manufacturer recommends.