We are in the process of consolidating oils and need to change from a xxx brand ISO 680 to a yyy brand synthetic ISO 460 in a worm gear box with a 5 gallon sump. What is the best practice for flushing out a ISO 680 oil from a worm gear box without disassymbling? Is it possible to flush this heavy oil out completely though the fill port and drain plug?

You could consider draining out what you can, and then flushing with a thinner oil (pref of the same brand/type of the new oil). If your gear box can run on both ISO VG 680 and ISO VG 460 you should not experience any problems with traces of the old oil in the sump (viscosity).

When your job i done, take an oil sample for analysis (from the gear box). The viscosity should give you a nice indicator on how it went.

..and, try doing this job while the oil is warm.

if PG - Then compatibility is an issue- How bout draining when the system is just stoppped and bit warm and flush by a flushing oil of ISO vg 32/22 and rinse by the desired oil and finally change over to the desired grade.