I've got a couple of Case lawnmowers that are hydraulic drive and speced to use 20w40 oil. Since that grade of oil is not an easy one to find I use Rotella 15w40. It is also speced to use 5w20 in winter with temps below 32 degrees. Since I use a snow blower on one of them in temps well below 32, would there be any issue using Rotella 5w40 year round? I'd like to change my oil change interval to once a year from twice a year.