Michael---The "book" on the TR engine calls for 10W/40 to 20W/50 when
the temperatures are 14 deg. F or higher. This same source says to go
down to 5W/20 or 5W/30 when below 14 deg. F. This is found under
"Recommended Lubricants" in the Triumph Repair Operation Manual.
So it looks like you can run the 10W/40 already in your garage, when the
temps are favorable. If the engine should leak more than you like with
the "lighter" oil, you can either use a viscosity thickener, such as
STP, or go to the 20W50. Any major brand should do here. I think all of
the modern oils are rated better than those which were available when
the TR6's were built.
Dick
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