The main difference between AW32 and AW46 hydraulic fluid is the weight - AW32 is a 10-weight fluid, and AW46 is a 20-weight fluid.
AW32 is a good all-season fluid best suited for use in cooler outside temperatures during the fall, winter, and early spring months.
AW46 is designed to handle the summer heat and in areas that don't see much of a seasonal cool-down. If you use this fluid, be sure to switch back to AW32 when the outside temps start to drop.
To learn about how much fluid to put in your log splitter, check out this article.