Motul SAE 50 is the heavier of the straight-grade options, a single-viscosity oil with no multigrade additives. It suits older and vintage engines specified for a heavy monograde, air-cooled units that run hot, and engines with generous or worn internal clearances where a thick film at operating temperature is what maintains oil pressure. Because a monograde is thick when cold, it is best suited to warm-climate use and engines that are properly warmed before being worked, which is precisely how most classics are driven anyway. The 2 litre size makes it easy to make up a non-standard sump capacity. Choose the 50 over the 30 only where the manufacturer, or your engine builder, calls for the heavier grade.