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SAE 50 motor oil is the right thing to use, but not because SAE 90 Gear oil is too heavy. There are two overlapping SAE scales: SAE Motor oil, and SAE Gear oil. SAE90 is about the same viscosity as SAE 50 Motor oil (Viscosity (100F/210F): SAE50 = 270/19.5 cST, SAE90 = 285/20.5 cST)...

If the SAE gear oil numbers were extended down from their minimum of SAE 75 to SAE 50, the viscosity of the resulting oil would be about the same as SAE 5 engine oil.... in other words, real thin stuff... I bet it isn't. The so called SAE 50 gear oil is just relabeled SAE 90 gear oil. -Chuck

There are two SAE oil ranges: one for SAE motor oil, and one for SAE gear oil. The ranges from SAE 0 through SAE 50 are motor oils, and the ranges from SAE 75 through SAE 250 are gear oils. The viscosity, or thickness, of the oils in the two ranges are comparable... with SAE 75 gear oil...