The PT-26 used either the Ranger L-440-3 or the L-440-7
Assuming you have the L-440-3 since it was the more common engine, the recommended oil viscosity is Saybolt summer grade 120 sec. and Saybolt winter grade 98 sec. - this falls into the SAE 5 weight range.
Best reference is the Technical Order 02-50AA-2 (1 January 1943)
If you don't have access to the engine manual and handbook, visit this thread where I have them posted:
Ranger Aircraft Engines