Howies551Ton,
Yes, the 58-63 USA 261s use a full-flow oil-filtration system. Deve plans on using an after-market spin-on-filter head on his 1959 (I think) 261 engine.
The filter, oil, and additives that might interest Deve should not be affected by any full-flow filter system (stock system or after market system).
Merry Christmas to you, too.
What is your temperature in Alberta (6pm on 22 Dec)? Our friends back home in Albany, NY are at 39 F and we are in NC at 65 F.
