Did you manage to check any engine fault codes using code reader (not the paperclip test)?
It’s also good to check all information about the engine – MAF sensor values, O2 sensors, engine temp etc.
What about spark plugs – do they look the same or are there any different colour? – you can check old ones.
I did the Paperclip test several times .. no faults shown (just 12 repeating). Please explain what is a "code reader" and what needs to be done.
All the spark plugs were replaced last Sunday using Bosch 520 FR91X Super 4 plugs, but it made no difference. The old ones seemed mostly OK .. no black arc scorch lines on insulators .. but a small amount of oil on outside, though Chris said not deep in wells.
Does it sound like I can avoid changing the rocker cover gaskets for now? I would prefer to know that the juddering has been resolved and the fuel economy has been restored before I incur further costs.