There is no equivalent.
Transmission oil Motorex, SAE 15W-50
Type: SAE 10W-30, SAE 10W-40, SAE 20W-40 or SAE 20W-50 Recommended engine oil grade: API service SG type or higher, JASO standard MA Engine oil quantity: Periodic oil change: 1.00 L (1.06 US qt) (0.88 Imp.qt)
Prescribed in the owners' manual is SAE 20W-50. The nearest best grade to use is SAE 15W-40.
They will mix together, but only use it in an engine if it is an approved combination of oil according to the manufacturer.
20W-50 is equal to Sae 90 W.
SAE 50 is a single weight oil that is 50 weight when cold or hot. SAE 20w50 is a multi-grade oil. When cold it flows as a 20 weight oil and therefore flows much easier. At full operating temperature it flows as a 50 weight oil. The W stands for winter.
oil type SAE 20W-50 for the 600cc engine you'll need 3.1 liters (according to the manual)
20w-50 indicates Synthetic-oil
Use exactly what the manufacture recommends as listed in your owners manual. Using anything else will cause engine damage.
Engine Oil; SAE 10W 30 to 15W 50 or API SG/CD Automatic Transmission; Dexron llD
sae 20 - 50
SAE 20W-50 Synthetic Motorcycle Oil