15w40 oil carrying both C and S ratings is great choice for tractors. brand almost doesn't matter. just about anything produced now is better than wat was around in the 60's. if you are in an arctic area.. I'd jump to a 10w30 c/s rated oil. or better yet. a 5w40 synthetic.
if she uses the big canister with element.. I am pretty sure it is a ch14pl fram. ( and crosses ).
if it is a spin on.. it is almost for sure the dual 300, motorcraft fl1a, fram ph8a, supertech st8a, napa 1452, 1515 etc... dozens of crosses.