dear friend, how to choose 387/4c87 with 386 cpu? dx sx sl and speed? does am386-40 paired with iit 4c87dlc-40, green math 4c87dx-40? if i80386dx-33 accept these 387s mentioned above or should take -33 speed?
You can always use a 40Mhz FPU with a slower CPU. Using an FPU slower than the CPU is risky and not recommended.
Have a look at your Mobo and identify the CPU. Then, identify the FPU socket. Then, find your best deal at your favorite 2nd hand listing site. And of course, SX wants SX, DX goes with DX.
I.e. I own an IBM Model 55SX PC, and I also own some Intel 387 SX FPUs PGA68, but they were not socket compatible when I wanted to do the (pointless 😁) upgrade, as the PC was accommodating for PLCC68. So I've had to find a "New" one.
If the motherboard uses linked 386/387 clocks, 387 will run at same clock as 386, a faster 387 can be used, but will clock down to the CPU speed, a slower one would be overclocked and be unreliable of fail.
With separate clocks (jumper and an additional oscillator package required), the 387 can be run faster than the CPU - running a 40 at 40 with a 33MHz CPU, or you could accommodate a slower 387 at its rated speed