Yes, most street benches need at least some ongoing maintenance, but whether you should be oiling or staining mainly depends on the material.
Wooden street benches are the ones that really benefit from periodic oiling or staining. A hardwood bench like teak, iroko, or oak will gradually turn gray in the sun. That weathered look is not necessarily rot, but without protection the surface can start to crack, cup, or hold moisture. Oiling a wooden bench once or twice a year helps replace natural oils and keeps the grain from drying out. Staining usually adds pigment plus a little more surface protection, so it is a good choice if you want to keep a richer color or cover UV damage. In public or street settings, many parks and councils use a penetrating wood oil or semi-transparent stain because it is easier to recoat than paint.
Metal benches rarely need oiling. Powder-coated steel or aluminum just needs occasional washing with mild soapy water to remove grime, bird droppings, and road film. If you see a scratch that goes down to bare metal, touch it up quickly to prevent rust. Wrought iron or uncoated steel can need rust treatment and repainting instead of oiling. Plastic, recycled plastic, and composite benches are the lowest maintenance: usually a wash is enough, and they do not need oiling or staining at all.
How often should you do it? For a wooden bench in a sunny or rainy climate, a light clean and recoat once a year is realistic. If the bench sits under trees or near salt air, you may need to do it every six months. A simple test: if water no longer beads on the surface or the wood looks dry and pale, it is time to oil or stain.
The basic routine is simple. Clean the bench with a stiff brush and a suitable wood cleaner, let it dry completely, then apply a thin coat of outdoor wood oil or stain with a cloth or brush. Wipe off excess after 20 to 30 minutes so it does not get sticky. Avoid heavy film-forming varnish on street benches because it can peel and become a bigger job later.
So yes, if it is wood, plan on oiling or staining as part of ongoing maintenance. If it is metal or composite, your main job is mostly cleaning and the occasional touch-up.