Thanks to recommendations on this list I have used & will continue to use Mann filters & Castrol GTX 20W50 in my Westy 2.1 original engine. Fram filter with 10W30 oil that came in the van when I bought it caused the oil buzzer to come on repeatedly. Never a problem since changing. 180k miles and going 'strong'. 

I am using the same oil/filter combination in my 1980 Triumph TR7 engine (the same filter fits exactly), immediately solved bad valve noise (though no hydraulic lifters in this engine) at start-up, another low oil pressure issue.

Mike
Palo Alto, CA
87 2WD Westy 'T3WESTY'