Generally speaking:
* Conventional oil: Every 3,000-5,000 miles is a common recommendation for conventional oil in a 1999 E-350. The shorter interval (3,000 miles) is more conservative and better for the engine's longevity, especially if you do a lot of stop-and-go driving.
* Synthetic blend oil: You might be able to extend this to 5,000-7,500 miles.
* Full synthetic oil: This allows for the longest intervals, potentially up to 7,500-10,000 miles, but always check the manufacturer's recommendation on the specific oil you are using.
Crucially: Always consult your owner's manual. It will have the manufacturer's specific recommendations for your 1999 Ford E-350 van, taking into account the engine type and intended use. This is the most reliable source of information.
Ignoring the recommended oil change intervals can lead to significant engine damage and costly repairs. It's always better to err on the side of caution and change the oil more frequently if you're unsure.
