For a car with low annual mileage (e.g. 4000 miles/year) that's mainly used for short, around-town driving and the occassional weekend trip, I've been following a time-based drain interval (every 4-6 months) instead of a mileage-based drain interval. I hear a lot about how the main benefit of synthetic is being able to go 7.5K-10K miles between changes vs. 3.5-5K with a dino oil. Would this extended drain interval also apply to a time-based drain interval? In other words, if I switched to synthetic, can I change it out every 10-12 months instead of every 4-6 months? P.S. Manufacturer recommendations for this car are: Normal: 7.5K/6 months, whichever comes first. Severe: 5K/4 months, whichever comes first.