N.C.T (23)

a leak in the cat will indeed cause mixture issues, but it will effect lambda mainly, and c/o secondly...
now it does for me to say car repairs are not an exact science, and you can look around for exceptions that break the rule, and they will be there, but the above tennant 'generally' holds true...

over-revving on cars with ecu / sensor goverened engines can most often be traced to temp sensor and map sensor issues, and by that i mean sensor, wiring (feed and return) or ecu and/or throttle body and stepper faults, and very occasionally, water in upc, as in megane, but im being type specific, a toyota, a vw or a ford can have their own querks and peculiarities...

over-revving on cars with cable accelerators (as opposed to fbw) and machanical throttle flaps (as opposed to steppers) can have freyed cables, and physical obstructions causing the problem...
though it is worth remembering cars as old as the midd to late 80s were incorperating biothermal spring and electro condutive technologies for engine temp regulation, and hence idle speed, (ford vv carbs with auto-choke are a good example), so again, there can be further checks required

but as i say, there are plenty of curve balls in repair and diagnosis, and todays cars now rely on eobd for after market service, and in house programmes for accurate assesement of faults within dealerships...the old ways are just that im afraid, old and gone...

and id better close with, i dont know it all, but i have spent most of my working life fixing cars, vans and commercials , and all of the above is based on practical experience...

sorry lad, dont understand the bit in red,

if you didnt overfill it with oil, id say two things, fuel and additive ratio was to low, but still, the additive is spirit based, so not likely to be the root cause, but if i read you right that your getting smoke at 3000 rpm, (probbo on acceleration), its burning oil either through valve stem oil seals or rings, but considering its low milage and your not complaining of a drop in power or 'lumpy tickover', im gonna stick my neck out and go with seals....

sometimes faults occour without any notice and we attribute it to something we or somebody else have/has done...

anyways, if you have a trustworthy mechanic, thats half the battle won...