whats wrong with that?

 

I did not mean that 50°F ambient temperature was cold, but instead that the engine was cold as opposed to being up to normal operating temperature.

It could be 90°F, and starting a "cold" engine and letting it idle for 15 minutes would be bad; or for that matter just letting it idle for 15 minutes at all. That's severe duty, like cabs and cop cars--making more frequent service mandatory, not just a wise thing to do.

Best service for any engine, at any except very extreme cold temps, is to start it, wait for the idle to stabilise, and then drive gently until the engine is hot. If you commonly take only short trips (< 10 miles) then once week take the car out for a 20 mile round trip at highway speeds to boil the combustion by-products from the oil.

If you cannot do that then change the oil according the 3 month schedule, not by mileage--that's what keeps cabs and cop cars routinely lasting 200k+ miles.