Grease or not grease, your choice. But a couple of things to consider. 1) most telescoping booms use UHMW wear pads, which is a slick abrasion resistant plastic, they're already slick, don't really need to be slicky-er. But hey, if you wish to make it slicky-er, again, your choice, but that brings us to point 2), you're only slicky-er-ing the outer boom wear pads, you're not, and can't, lubricating the inner boom wear pads. I suggest you inspect your wear pads and see if they need replacement, especially removing the rear cover of the boom and inspect the inner wear pads with the boom fully retracted.