Oil pressure warning light keeps flickering - what could be causing this problem?

I'm a London taxi driver. My cab is a 2006 TX 2 that's done about 234,000 miles on the original engine. Recently, the oil pressure warning light has started to flicker when the engine has warmed up, more so on a bumpy road. I've had the pressure switch replaced and my garage has tested the oil pressure and assures me that it's okay. The cab is regularly serviced and between services the oil and filter are replaced i.e. every 5000 miles. Before the flickering started I had taken to adding a diesel additive containing powerful detergents to clean the injectors and make the cab more economical. I have now stopped adding this to the fuel. Any idea what is causing this warning light to flicker please?
Despite your very sensible 5000 mile oil changes, sludge may have built up in the sump and slops about, sometimes partially blocking the oil strainer. If the oil was changed by siphoning the old oil out rather than draining it via the sump plug I'm more likely to be right.
Answered by Honest John on

Ask Honest John