Google search found a good website that has a blend calculator. http://www.lubebase.com/

Let X be the fraction of the mixture which has VG 320

Then (1-X) is the fraction which has the original VG 460.

X*320+(1-X)*460 = 360

320x+ (460 - 460X) = 360

100 = 140X
X = 100/140 = 5/7

If the total volumne is 23.5 gallons, then the volume of VG320 would have to be
X*23.5 = 5/7 * 23.5 ~ 17 gallons

Thanks. The only issue is that there is no work history of an oil change/additions on these machines and that is quite a bit of oil to add even over an extended period to get the viscosity to where we see it. Since we changed the oil to the proper grade I guess I will chalk this up to one of those strange mysteries that happen in power plants that are never explained. I guess I won't spend any more time investigating.

We have a similar issue with a PG320. The problem happens in most of the gearboxes we use it. We keep getting sample reports with VG reduction down to about 260-280. We had a go at adding 460 to bring it back up but only with guessing the amount. These machines have been running for 10 years. Due to group procurement agreements we have to use oil from a certain company. With the two previous companies we used there was no such issue with VG changes. Any thoughts on what could cause these VG issues?

See table 5 here
http://machinerylubrication.co...4/oil-analysis-tests

hello rgf12:
i try to understand your issue,gear oil 's viscosity(320) drop and you could add new oil with high viscosity(460) to balance it.

is my comprehending correct?