How many cord a year are you planning on? The biggest thing I've seen cause premature wear is to have your slide either too loose or too short. the longer the better as it will take a lot of the side flex out of the equation IF you have everything close and smooth to start with.
I'm running a box beam and box slide and it is 16 inches long. It's not going to wobble side to side. Make sure to smooth everything up and don't let ANY weld slag or burrs left on there. A burr can create a drag that will start a gouging action and wear grooves.
Notice how mine looks black here? That's the following.
It's Graphite paint. Also called slip plate and can be applied with a brush. Most Ag supply stores or tractor dealers have it. Farmers use it to paint wagons on the inside to get grain to flow. Used to use it on combines and plows to keep the rust at bay during downtime. It's so slick you'll go head over heels in a grain wagon if you step on it. It wears off quickly on high contact points like anything does but I found it helps a lot. Same deal, paint it if you know it's going to be setting for a long time.
This is the first year after about 75 cord worth of wear.
I wouldn't use grease just from the mess factor. You'd have it everywhere and as stated, it'll hold the dirt there, what you don't want.