I’ve got a 1952 Farmall Super C. I run Rotella T4 10w30 in it. This oil has HTHS of at least 3.5 and has higher levels of ZDDP for antiwear. If I were to work an antique tractor hard in warm weather I would step it up to a 15w40. Either of these are thicker and higher in ZDDP than typical sae 30...