I'm fairly sure I've replaced all my OEM springs by now with Titan ones, I like them for their good consistent quality, but they are sold over size so you've almost certainly got to cut them down, which can be a problem for some I know. There's a fair bit of experimentation to get it right, you can't cut it back on if you go too far.
And that black low friction coating makes some pretty lethal smelling fumes so its best not to collapse a coil on the gas cooker in the kitchen...
I did try a square section spring once in my Airsporter, Pre chrony days, but I'm pretty sure it was producing less power for more cocking effort, & after a week I took it apart to find it full of swarf where the spring had been gnawing at the inside of the piston.
I've lost track of it in the last 40 years, which is a shame because I currently need a gate closer.