Magnatec 10/40 oil

[Chris KC]

I was being slightly flippant & agree a proper oil change is desirable, I wouldn't do it more than once a year though with multigrade oil and a full flow filter. Unless running in, in which case an earlier change is called for.

[Bob Culver]

Myriads of older cars without filters have been run on post mid 1950s detergent oils without problem. In many caes after API SB and similar basic oils. It used to be claimed that detergent oils reduced future deposits but did not substantially loosen existing. Dunno if that applies to API SL and similar exotics.
A few years ago a senior public service engineer here got into trouble when he compared the use of CNG for electricity generation to "feeding chocolate to pigs". I suspect the same applies to many oils fed into Sevens.

[David Stepney]

A filter is a must with multigrade oil as it is designed to keep the dirt in suspension so filter can remove . Straight oil drops it into.bottom of sump or oiltank as on classic bikes .

Exactly! That is why, without a modern filter, regular oil changes are are a must.

[Smiley]

High folks,  Thank you for all the replies.  Looks like a trip to Halfords.  
Cheers.

[Bob Culver]

The VW Beetle with bearings less absorbent than white metal, ran with no filter and 3,000 mile changes at least until the early 70s. In 1970 API SE was the standard; not as detergent as later but early cars would have been run with lower detergency oils and changed to SE. In all  cars current more detergent oils would have been used in following decades. I dunno whether the very late South American production had filters. 
With Sevens a magnet seems desirable to collect the timing gears.