Most oil boilers can be serviced in an hour. A new nozzle is recommended each year. Oil hoses used to be notoriously brittle under the braided outer and leaked after a couple of years. "Long-life" hoses became available which should be good for five years or in many cases far more. Consumables such as fibreglass rope, fire cement and high temperature adhesive weren't items I charged extra for. "Washable" strainer were common rather than cartridge filters. I operated an annual service rota for my customers and normally could arrange a series of close together calls which allowed me to keep prices reasonable. By way of comparison a service to a domestic oil boiler in 2003/4 including nozzle was £65 in Yorkshire.
I often had calls for a service which on attendance were seen to be prompted by breakdowns. These obviously cost more depending on what parts were required. I normally carried a wide range of spares as van stock to avoid having to trail off to suppliers although some of my competitors deliberately carried no stock and charged for time and travel which penalised the client.