The manual transmission in my mazda 6 was quite stiff until I replaced the gearbox oil with ATF, much easier after the change.
Most manual gearboxes made since the 90s, call for ATF. I have changed to regular oil quite often. On my BMW e30 (4 cyl, quite rough running) I get a lot of noise at idle when the gearbox is hot and I use ATF. It is due to backlash in the gears and the rough running engine. When I use regular oil it becomes much quieter.Only downside is that in cold weather the synchros aren’t as effective as they are with ATF.