Just had oil changes in my 2010 Nissan Sentra and it is black already. It's only been about 3 weeks.
That sounds like you do a lot of short trips and have some build up inside the engine, you could have the engine flushed out but this could cause some seals to start leaking or you could try changing the oil and filter more frequently until the oil stays cleaner longer. I have always changed my oil at 3 months or 3000 miles what ever occurs first. Your manual will state than around town driving is considered severe duty, so change your oil sooner.
I'd recommend putting synthetic oil in your SE-R. If you have very cheap oil like Walmart's Tech2000 (which I've heard is recycled/filtered/re-refined used oil), you will not get the expected mileage. This said, get your oil tested if you suspect something is wrong. You'll know if there are debris in it or if your it's just burned up.
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