it was already changed when i bought it and both were manufactured when my car was made (1994)

Use your VIN number, go to www.realoem.com, once you key in your VIN, it should already list (highlighted in blue) your engine type. This website is also fantastic for you to look at parts, part numbers and estimated prices.

The key difference between the two engines I believe is:
The M40 uses a timing BELT. The M43 uses a timing chain. There's also a slight difference in torque figures (6lb-ft).

Another way to find out is simply calling the dealer and giving them your VIN number.