Unless you run them heavily, once a year is enough. Get a good plastic compatible oil like Labelle's #106 with a needle applicator. One drop to each bearing is enough, too much and you'll be gathering dust with it. A very small dab of plastic compatible grease should then be applied to the teeth of the gears. The older the loco, the more likely the grease and oil have gelled and caked, so a thorough cleaning is in order. Disassembly is intimidating but not impossible. Start with your diesels first, gradually working up to steamers. Fortunately in N scale, steamers are usually built much the same way as diesels---rarely do you have to do anything but lube the siderods. Some careful quartering may be necessary on steamer drivers but don't let it intimidate you. And ALWAYS refer to your exploded views on your instruction sheets. I recommend a small muffin tin with each cup numbered sequentially during disassembly. Place each part in the tin in the order it comes out, then reassemble in reverse.