Found a new one to try instead of Halfords.. https://www.classic-oils.net/Classic-Oils-Heritage-20W50 £ 19.95 Classic Oils' own high-zinc classic 20W50 mineral oil with 1300ppm ZDDP (5 litre, 25 litre) And for those that like their numbers.. I know you're out there.. Detailed Specification: Specific Gravity @ 15.6C: 0.889 Kinematic Viscosity @ 100C: 20.88cSt Kinematic Viscosity @ 40C: 189.03cSt Viscosity Index: 131 Flash Point COC: 235C Pour Point: -31C Total Base Number: 10.2mg KOH/g
Lucas classic hotrod 20/50, or10/40 has high zddp.(2100) Or their 10\40 semi synthetic (1265ppm).#10176 Mobil 1 extended life/high mileage 10/60 costs bit more....but any leftover is OK for neighbour's 911.(1300ppm). Not sure who makes halfords oil, or even the staff there, so no brand name to protect there....
Motul (not mobil) oil which is well rated in motorbike / racing forums has two new oils out. Called 'Eighties' 10w40 it boasts high zppp(1800ppm) and is semi synthetic. I have not tried it yet, so let everyone know if you find it ok. If. I can't get amsoil premium protection 10w/40 that's my next choice. Maybe 20/50 is ok with non hydraulic tappets, but I doubt there's much in the way of up to date protection and is made to keep Morris minor/mgb owners happy. Even mid eighties I did not see it at vw main dealers.
My engine has very good oil pressure, but I found 10w40 was worryingly slow to build pressure where 15w40 is almost instant.
I use this I found the engine ran hotter with 20/50 I think it’s because the pressure was sat around the the closing pressure of the the control valve to the oil cooler.