My fd has 89k on it. The oil pressure is real low. Nearly 0 at idle and 30 at 3000rpm, it also has a knocking sound when It idles and the oil pressure low. SHould I replace oil pressure sending unit. Is it expensive and or somthing I can fix at home?

 

Mazda oil sending uinits suck and this has been know for decades! They go bad very easily, and show low or intermittent low pressure.

Now the knocking noise is something seperate and most likely bad.
If it occurs at about 1/3 the rpm rate, then you have problems with: too loose a rotor bearing as is worn out, rotor to housing slap also caused by the previous, or loose oil cooling jet from the e-shaft bouncing around inside the rotor.

Verify the oil pressure first. If it really is low, time for a rebuild.