Hi,

I have a 93 chevy 3500 6.5L turbo diesel with about 215k on it. Some smoke on start up but other then that its been running fine till recently at about 40mph it loses about half the power and I can watch the RPMs dip down.

Changed fuel filter no difference.

Any other help appreciated...

most likely a bad ops or fuel pump do the checklist paying close attention the these items.

Open the tee valve next to the thermostat while the engine is running. If it dies you have a fuel problem.[lift pump not working] If pumping , the fuel cap could be gunked up and not venting the tank[try running with the cap loose] Another suspect is the fuel pump relay[under the black plastic cover on the passenger side firewall] You can unplug the relay and jumper the big red to orange[if I remember correctly] to run the lift pump. Make shure the relay is good before condemming the lift pump or oil pressure switch.