@Rickw2 since the gears in your 763 are metal, you could use pretty much any grease with no risk of causing harm. Lubriplate, automotive moly grease, etc., would all work. The Labelle is especially good for nylon gears because it won't melt them or make them get gummy.
By the way, looking at your photo... is that a crack in your chassis running across the gearbox? There has been a lot of discussion about zinc pest on the forum. I wasn't aware of any Lionel pieces from the mid-'90s that were affected, and for your sake I hope it's not the start of something very bad!