The SX 18 is pin for pin compatible with the PIC16F54. The SX28 is probably same with PIC16F57.
SX are clones of PIC's. At the time I guess they never thought there would be a problem with Microchip but then there was.
There is a company called Holtek that is still to this day clipping along with their PIC clone and looks like Microchip never bothered them about it.
SX has an RTCC at pin 3 and PIC is something else.
The Port pins all match up though.PortA 0 to 3.Port B 0 to 7. VDD and VSS pins match up.
If you want to program the two in circuit that's a problem. They use different connections for programming so that limits actual swapping into a circuit.
SX manual is a little more plain talk than Microchip datasheets.
SX and PIC assembler are very similar.PIC has 35 instructions, SX a lot more.
Very interesting to compare the two whichever one you're using.