Hi, we have an "Airco" brand furnace that dates to the 60s. It's well-maintained. However, it has just started to emit an erratic, loud squealing noise. We changed the belt (and filter) to no avail. We cannot see anywhere to add oil. Is there anything else we could try? Thanks!

You said that you have changed the belt, so if it has a belt then it has bearings on the blower wheel. Unplug the electricity to the unit and with a flash light look in the furnace to the wheel that the belt turns the blower, about two inches there is an oil hole for you to put oil in. There is also one on the other end of the shaft and it might be best for you to try to find this one first because the one on the other end is hard to see. Squirt oil several times into each hole, (the hole may be covered with a little spring loaded lip that will lift out of the way so you can get an oil can nozzle into them.
Now if it is making a very bad squealing noise it might be to the point that you need to change the bearings, they are probably Pillow block bearings and can be purchased at any good hardware store. The easiest way to replace them is to pull the blower out of the furnace. The sounds hard but it is fairly easy, just hard to find the way. There will be a couple of screws somewhere that will not let it slide out of the furnace, find these and take them out, the pull the blower forward. Take the wires apart, or cut them and mark the wires so that you will get them back the same as they were. The shaft is a little problem getting it out but can be down if you just look at things to see how they work. For instance there is a set screw that holds the blower one the shaft, it will hae to be loosened, If you have to replace the bearing at the end where the pulley wheel is you have to take the shaft all the way out of the blower, and there is a spacer lock clamp that holds the wheel centered in the opening... Go for it you can do it just watch what you are taking apart and put it back the same way... take the old bearings with you to buy anyone's because they are different kinds.