Just a point someone mentioned to me once? When a 80w/90 spec oil is specified why can`t a 75w/90 oil be used as it covers the 80w range as well? And will save you getting another grade of oil?:scratching_chin:
It will possibly be slightly too thin at certain temperatures.
Having said that - fresh oil of marginally the wrong grade is almost certainly going to be better for the car than the old oil that is in there already.