Regarding your lawn tractor oil: I would again recommend the use of the specified SAE 10W-30. Using the SAE 10W-40 means that it has more viscosity index improvers added to make it suitable for the wider viscosity range. The extra improver additive has been known to cause piston rings to stick, leading to increased oil usage.
Staying with a known, brand-name SAE 10W-30 and using a synthetic one is a wise option, in my opinion. Change the oil in the lawn tractor as specified by the manufacturer, which is usually every 25 hours of operation.
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