Mike, ALL Colt, Case and Ingersoll garden tractors, including the 600, 6000 and 7000 Series loader and loader/backhoe models use the exact same oil.
I prefer 20W50 MOTOR OIL because the factory recommended 20W40 is nearly impossible to find and when you do find it, you are paying a premium price for it. An alternative to the 20W50 is 15W40, which is a grade that is very easy to obtain. Shell Rotella or Chevron Delo are excellent choices.
You can also use this oil in the trans-axle.
In order to get your tractor to perform properly, you are going to have to remove the hoses from each hydraulic cylinder and disconnect the cylinder rod from the loader so that you can pull that rod all the way out and then push it all the way in to force out all of the oil.
This procedure must include the mid-lift cylinder and the 3 point hitch cylinder. Just refilling the reservoir alone will leave 50 percent of the WRONG oil in the tractor and you would have to drain and refill the tractor about 10 times in order to reduce the percentage of incorrect oil to the same level as you will achieve by manually draining all of the cylinders NOW.
The oil in these tractors is the same as the driveshaft in a car. It is the final link that transmits engine HP to the rear end and subsequently to the back tires. The irony here with the oil is that it is NOT about the performance of the loader because the loader cylinders do not care all that much about which oil is used. Many loaders specify ATF, which is only about 5 weight when compared to other motor oils. It is the drive motor on these tractors that demand a motor oil that is at least equal to straight 30 weight when at operating temperature.
True hydraulic oils do not use the same viscosity table as what motor oils use. These tractors are just one of many, many machines that specify motor oil for the hydraulic system.
You will be doing yourself and your tractor a major service by removing as much of the oil oil as possible but there is no need to get crazy over trying to get out the last quart of oil from the hydraulic lines, oil cooler and so forth. Any remaining oil will mix with the new oil for now and when you drain/refill the reservoir 2 years from now, the percentage will be reduced even more.