Well,
Re: Magnatec:
The negatives seem to be in the area of noisy lifters. Here in Australia, the Magnatec is 15/40, same as UK, but when I installed Magnatec in my Ozzy Mitsubishi V6, guess what? I suddenly had noisy lifters until the engine is revved above 2000 rpm on startup. So, conclusions are that 15/40 is fine for UK but for Australia perhaps the viscosity of Castrol Magnatec should be 20W/50. Having said that, once the lifters have filled up, there is no more noise. Maybe Castrol are missing a trick down-under.
As for the dipstick, I can confirm that there is no magic dipstick, and they must be calibrated. I have ground two small nicks in mine, one for correct, and one for correct minus one litre, just to give me something to aim between!
Tony.