Fergus wrote:have you 'any' first-hand information yourselves concerning this by doing it to an early Mx5 - NOPE !! You Don't !! But I do .
actually , yes fergus i have.
once upon a time 9 yrs ago i bought a fresh import rs-ltd 1.8 1994 with 64000kms on the clock
registered it , taxed it and drove it for a few months
then one day i decided to change all fluids and in my infinite wisdom i decided to run mobil 1 fully synth
after a full service i noticed the engine oil pressure was real low, and the engine sounded a bit clattery
a week later i had oil leaking from every possible place that it could leak from and to top it off the oil pump decided to lunch it self
on removal of the oil pump the pressure valve had popped
Spoke to a mazda tech and the first thing he asked was -" have you changed to a thinner oil ?" - thats your problem right there he said.
1 new oil pump later and some 10w40 semi synth and the car was fine
subsequently the car was written off a few months later but i know where the engine went to and is still in service to this day in a 1996 s-special type 2
thats just my experience for what its worth ,however, others may have had a more positive experience