When you can't find the correct user manual you go by intuition. You unscrew all lids and plates (needle plate, shuttle plates, face plate, and the bit in the front), and clean up as much dust, treads, fluff, old oil and grime as you can. Then oil all hinges, gears, every part that moves where metal touches metal. Lift up the machine and look under the base, there's lots of parts that need oil and cleaing. Where there are small holes in body, the bobbin winder and in the metal (gears and hinges), they are most likely oiling points. Behind the face plate all parts need oil, where the presser bar and needle bar goes through the cast iron body and perhaps steel shafts. Oil repetedly, lift up presser bar and turn the hand wheel now and then. Keep it up for a few days, it can take a few days for oil to reach the inner most nooks and crannies and flush out old sticky oil and grime.
The knob above the bobbin winder is the stitch length adjuster, turn it in and out to its' extreme positions while you oil the threading. You can with advantage oil generously until parts run smoother. Before you start put a rag or paper towels in the botton of the case to pick up any oil dripping down, it likely will. After a bit of use brown oil and grime might seep out of joints and you have to do a second round of wiping of and oiling. Eventually the oil will stay clear and clean, and you can get more relaxed with the oiling part.
You can with advantage polish up the shiny metal parts with something like Autosol or Quick-Glo, or similar. It's often important to oil, clean and polish up the bobbin winder of this type throughly. The long bobbins need the thread to run very smoothly on the mechanism to wind smoothly.
You can carefully clean it with microfiber cloth and regular sewing machin oil, it's the most common advice. I have used a slighly damp microfiber cloth with out any damage to either decals or paint, not everybody trust this method though. Be carefull with the decals, and take care for them not to rub off. They usually can take a gentle cleaning and polishing, but if they are damaged and brittle they might rub off. Yours looks quite good though, a bit of wear to them, but that's to be expected. Take your time and gently go over the machine with circular movements, all dirt and grime will come off. You probably have to use a few cloths. I tend to use a small pile of cotton swabs too. I polish my machines with hope that it leaves a protective film peventing further wear and tear to the finish.
If you do this, the repair guy will have a much easier job when he tends to missing motor belts and replacing electrical wires.
You machine looks very nice, not too far from a Singer model 127.
This video is for a Singer, but I think it's similar enought to be helpful. You might need to improvise a bit along the way, but the general way about it should be there. Do you have both the sliding covers for the shuttle or is it missing?
Edit, I found a
video on a Domestic too, this guy has a bit of information as well as a demostration.