You probably don't need an oil change every 3,000 miles

The 3,000-mile oil change is one of the most profitable pieces of misinformation in this industry, and plenty of shops are in no hurry to correct it.

It was right, once. Conventional oil in a 1985 engine with loose tolerances genuinely broke down around there. Then oil chemistry improved, engines got tighter, and manufacturers started publishing intervals of 7,500 or 10,000 miles. The sticker on your windshield didn't get the message.

What actually decides it

Three things: what oil the manufacturer specifies, what the manufacturer's interval is, and how you drive.

That third one is where most people get it wrong, and not in the direction you'd expect.

You're probably on the severe schedule

Open your owner's manual and you'll find two maintenance schedules. Normal, and severe. Almost everyone assumes they're normal driving.

Severe service means short trips, stop-and-go traffic, extended idling, towing, or cold weather operation.

Read that again. Short trips. Cold weather. If your driving is Graham to South Hill and back, a school run, and a trailer a few times a year, you are on the severe schedule, and your interval is shorter than the number on the dashboard reminder.

Why short trips are the real problem

Every time a cold engine runs, combustion puts water vapour and small amounts of unburnt fuel into the crankcase. On a long drive the oil gets hot enough to boil that off.

On a five-mile trip it never does. So it accumulates. Oil that's been through fifty short trips can look perfectly clean on the dipstick and already be past its useful life, because the problem isn't dirt, it's dilution.

This is why two identical cars with identical mileage can need completely different service intervals, and why "it still looks clean" is not the test.

So what should you actually do

• Use the oil grade your manufacturer specifies. Not a heavier one because someone told you it's better for an older engine. Modern variable valve timing systems run on oil pressure and they're calibrated for a specific viscosity.
• Follow the severe schedule if any of the above describes you. It usually does.
• If your car has an oil life monitor, trust it. It's watching engine temperature and load, which is more than a mileage number can do.
• Don't stretch full synthetic to 15,000 miles because a forum said you could. The oil might survive it. The filter won't.

What we'll tell you

If you come in at 3,000 miles and your car doesn't need it, we'll say so. That costs us a sale and buys us a customer, which is a trade we're happy to make. Here's what an oil service with us actually includes.