Sandy,
Some people mentioned in posts that the D4000 did not require grease in the lead screw while the rest of the scanners do. I would lubricate the lead screw, not matter which model, with a plastic/rubber safe grease. Aztek sells their recommended grease in their online store. I use (not a recommendation) a grease called Extreme Fluoro for the lead screw and the tailstock bearing, available in some bikes stores and online.

Look first under the cover because sometimes the old grease dries and form almost a chunk of "Cheddar" that needs to be removed.

For the rails check the nearest Lowes or online for "teflon silicone lubricant". It is also safe on rubber. Another is pure fluoro oil, also from Finish Line. But the most important part is to do it several times, cleaning the dirt from the rails, applying the lubricant, move the carriage to the right (using Test menu), repeat the operation on the left side, 3 or 4 times.

Believe or not many owners are not aware how important is to keep the scanner lead screw, rails and bearings clean an lubricated. It not only shorten the life of the mechanical components because of rapid wearing. It also put a lot of stress in the electronic circuits driving the parts.