S.A.E. viscosity (grade, weight) is, by definition, thickness (flow resistance). It's derived from the time in seconds it takes for a given quantity of liquid to pass through a sized opening (not lubrication properties). Sometimes the S.U.S. (Saybolt Universal Seconds) standard is used instead, as in Harley-Davidson oil rating; 105 S.U.S. is more or less equivalent to S.A.E. 50.
A multi-grade oil's additives artificially thicken with rising temperature: 30 @ 212° etc.
I'd try 10-30, but if it doesn't pick up at cruising speed the oil isn't the problem.
