My new plane is a Midwest Skyduster and the elevator pushrod binds a little through the range of its motion. It is a hollow plastic tube with threaded rods screwed in at each end which is fitted inside a larger plastic tube that serves as a guide. Can I use oil or grease to reduce the binding problem?

Tempus, a small amount of binding is inherent in the type of pushrod you're using. If you feel it's excessive, check the radius of of any curves, look for irregularities on the inner rod and make sure the links from exit to control horn and servo are straight and not causing side loads. Talcum powder is a good lube if all else is normal.