Yeah I've use M1 for years. Car was a fleet vehicle for 3 years before I got it, and did need the vacuum pump seals done soon after. It's been fine ever since with M1.

As have all my other cars... an Acura had been run on dino oil and the valve cover was covered in crap. After about a year and 20K miles the M1 cleaned that up to a shiny new appearance and also improved highway mileage by about 30%.

I've been absent for a few days, so to catch you up on the progress...
I've done about 120km since replacing/fixing, and everything seems ok. Engine is running smoother than before (probably the better quality oil), and the gearbox is also noticeably smoother (I'm sure it's the first oil change since new). Whereas before, there was quite a change when shifting from 1st to 2nd, you now only notice it by the rev counter dropping.
The oil light persists when warm, and I haven't had a chance to get it onto a diagnostic machine - dad's been in hospital, but I will get it booked in next week.
Regarding the oil: South Africa is a pretty warm country, and although I am at the coast (literally a couple of 100 meters), the winter daytime air temperature is around 15 degrees, while summer is regularly well over 30, sometimes topping 40. That's 60F in winter and 90to 105F in summer. That, coupled with the miles on the motor, was why I had opted for the 10W40 prior to this oil change. With the 40, I never had an oil light on, but with the 0W30 I now have in the engine, the oil light is showing. I think I'm, going to run this oil for another 200km, maybe 300, then drop it and go for a 40W.
A few minutes ago, I did another flush of the cooling system, and was pleasantly surprised how little oil there was - just a light film on the surface of the water in the reservoir. Once more flush then it should be good.