Here is my final word on the subject.
Remember this info pertains to 5000 series units only. I did not look into any other units.
I just spent a pleasant 30 minutes on the phone with a tech support expert at Hydo-Gear. He was very informed and helpful. I am very greatfull for his time and help.
What Hydo-Gear looks for is a sustained viscosity of 55 minimum. His recomendation is to use 20w50 standard Mobile oil. If you want to pay extra then use the 15w50 synthetic Mobile 1. If units leave their factory with fluid in them, it is the standard Mobile fluid. Others may add additives but they are not necessary, according to him. The recomendation of Mobile is based on technical data. Shell TTF-SB oil was compared to the Mobile oil in our conversation. The Shell oil had some additives for slickness, not necessary in his opinion.
The oil should be changed every 200 hours. Not uncommon to have to initially change sooner. No matter how clean they try to be in the manufacturing process, sometimes dirt is still in the unit. The dark grey color comes from the oil circulating in an aluminum unit, not a cause for concern. It will eventually change to black. None of this is supposed to change the viscosity, which directly affects performance.
On the subject of oil filers. He recommend that you stay with the oem filters. Or at least be extremely careful when xref. They do not xref any oil filters.
I can not justify paying at least $9 a quart for oil.
I will be changing the oil using 20w50 mobil and using oem filters from Cub.
Hope this clears up some questioning minds out there. I am persistent, if nothing else.
Alan