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I'm not familiar enough with the Ruckstell to know if it has phosphor bronze bushings in it or not. The high extra pressure lubes rated GL5 will slowly eat away at the soft metal bushings if applicable. The mild EP GL4 is available in the Sta-Lube brand from NAPA and other distributors. The extra pressure additives are low enough as to not be much of a problem with soft metals like brass and phosphor bronze. If there are no soft metal load bearing parts in there, you can use any gear lube you want.
Another safe way to go is SAE-50 motor oil since it is equal to the SAE-90 gear lube viscosity. Aircraft and Harley Davidson motorcycles still use the heavy viscosity motor oils. Some aircraft oils have a pressure additive and they should be avoided.