In an ideal world the grease nipple drilling would pass through the bush into the internal surface channel of the bush. But that's not how it's made. The lower nipple should really be half an inch lower, and the groove in the kinpin which is supplied from the top nipple should also be similarly lower. That would feed the grease into the bush channels. As it stands, if you have an upper nipple the grease will emerge just above the top bush, so on compression of the suspension, some will find it's way down the bush. Remember that the upper bush doesn't take much lateral force - being more of a fulcrum. You could use oil instead of grease, but I doubt it will make much difference. The lower nipple is the main one - being above the lower bush means that every time you add grease, it fills the space between the bushes, and will force upwards and downwards between the bushes & kingpin. This is clearly why it's located where it is, and why you can get by with one nipple per side. Grease it little & often and it will be fine. Don't overdo the top nipple - it will only fill the cavity inside the dust cover and make a real mess.
