Many thanks to Mareng73 for that comprehensive reply.
Problem/resolution by you described exactly , I am sure.
The car was bought in 2018, 10k on clock , from Milcar Mazda , with a 12 month warranty and I wrote to the main Mazda Tech people ,explaining problem after RED light came on after 6500 miles , mostly motorway driving.
They blamed my driving and changed oil/filter under warranty.
Of course , it kept happening every 6500 miles thereafter , out of warranty.
Crooks , by any other name.
We never had these problems with the Sulzer RD90's , a proper engine !