Morris Minor Owners Club

The oil circulatory system of the 803cc only filtered some of the oil before it went to the bearings.
The oil circulatory system of the 948cc engine filtered all of the oil before it went to the bearings.
The fully filtered oil system as fitted to the 948cc engine and 'A' Series derivatives was and still is a much better system than that used on the 803cc engine.

Quite so. I wouldn't say that the oil filter "worked fine" on the 803cc engine. In fact, I wouldn't say that ANYTHING really worked terribly well on the 803cc cars and the 948cc models were a HUGE improvement