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Could that be a 42512x99A instead? Maybe an extra 5 in there by accident? I'm not finding your model in the database.
Thank you. That's a Tecumseh LTH Transaxle on there. In order to drain it, you would need to remove the transaxle, remove the fill plug from the top and drain it upside down. It's not designed to be maintained with oil changes the way an engine would be. You can, however, add oil to either the pump section or the differential section is it's low. Again, this is actually designed to be done on a bench with the transaxle removed. Here's some information that shows the procedure and what lubricants to use. Hope this helps!