Yes, you can use 10W30 in your riding mower. However, there are a few things to keep in mind before doing so.
First, check your owner's manual to see if 10W30 is recommended for your particular model of mower. If it isn't specifically called for, then you may want to stick with the type of oil that was originally recommended.
Second, be aware that using a heavier weight oil like 10W30 can make your engine work slightly harder than it would with a lighter weight oil. This isn't necessarily a bad thing, but it is something to keep in mind.
Third, remember that you'll need to change your oil more frequently when using a heavier weight oil like 10W30. How often you'll need to change it will depend on how often you use your mower and the conditions you're mowing in (hot weather will cause the oil to break down faster). Consult your owner's manual or a qualified mechanic for guidance on how often to change the oil in your mower.