A very old boiler that's open-vented (not pressurised) is likely to have quite a lot of black sediment in it from the steel radiators, unless it has been cleaned and had a filter fitted.
If not, I'd strongly consider having a Magnaclean or similar fitted - it will cost about £100 plus fitting - and a cleaning chemical circulated to loosen the sludge, which the filter will then trap. Ask the installer to show you how to empty out the dirt from the filter, it's as necessary and as easy as emptying your hoover bag.
Sediment will clog the circulation, radiators and boiler, and will wear out moving parts such as the pump. Sediment in a radiator displays by the middle, or the bottom rail, being cold.
X400 can be circulated for about 4 weeks before it loses it's cleansing power, it's supposed to be drained out with the loosened dirt, but if you've trapped most of it in the filter, it will not actually do any damage if you leave it in for longer. To drain it you'd need to access the small feed and expansion tank l in the loft, and the drain cock that will be downstairs, possibly close to your boiler or the radiator nearest the door.
Other more aggressive cleaners can work faster but have to be drained and rinsed out quickly. They may be acidic.
The installer can do it but will charge for her time, so if you can manage it, it's a worthwhile DIY job. It's less effective than a powerflush, but hundreds of pounds cheaper. Often it will prevent a blockage occurring and a powerflush won't be needed. If you wait until there's a blockage, it's too late.
Oil boilers are usually very durable and last a long time with reasonable care.