If the reservoir is dry then you probably have a leak somewhere and bleeding won’t be productive.
SAE 20 will be hard to find today. I would consider the much more common 10W-30 oil a good replacement for SAE 20. Or use a good hydraulic fluid. At least an AW rated oil, or a premium hydraulic fluid. Don’t use the cheap hydraulic oils, they have a minimum of antiwear additive, their levels being far below that of a good engine oil.The transmission and hydraulic system calls for SAE 20 engine oil. Isn't there something about the weight of oil from that time period is not the same as todays? Something to do with mineral oil?