Mobil one 15,000 mile oil is dark brown?

Is this a low mileage car, or one with many miles and worn engine parts allowing more combustion by-products to escape into oil system?

Is this a car that has had regular oil changes to preclude engine deposits (sludge) in the oil system, or one that was neglected and the synthetic oil is cleaning accumulated deposits out?

Has it always been run on synthetic or semi, or just now switched from conventional to synthetic?

Is car driven mainly at higher speeds or long enough for oil to heat-up completely and burn-off contaminants, or is all stop-and-go driving?

Of course, all detergent oils are supposed to pick-up and carry contaminants rather than allow them to collect on engine parts.....so oil changes color as it collects dirt. If your driving habits lead to early and unusual oil contamination, consider changing more often.

So depends on condition of your engine and driving habits. Another example: I have a 13 year old, 70,000 mile cream-puff . . . . always over-maintained, now driven less than 2000 miles a year but when it is, we take it out on the highway for a good run. For a while we followed the usual "whichever occurs first" scheduled maintenance, but the mechanic said it was unnecessary the way we use it. So now we change every 2 years, and it comes out as clean as when if goes in . . . the mechanic saves my drain oil for his driver !!

Dividing the cost of an oil change over miles, it will only cost you pennies to change more often.

Follow the guidelines in the owners manual. The guidelines for oil changes were written by the engineers that designed the engine. It is the correct schedule to follow when changing oil.
Two things make oil change color. Contaminates and heat from the engine. Your change could be from either or both. Another reason to follow the schedule in the owners manual.

I'd only go 10K miles. Synthetic oil is a high detergent oil, and it turns brown more rapidly that conventional.

Change your oil at 7500 miles.

Most mechanics will tell you to change synthetic oil every 5,000 miles along with the oil filter.
Oil filters have always been made to work for 3,000 miles and they get smaller every year! Dirty oil is not good for the bearings. I change my oil before it gets brown or black and never let it go below half way don between the upper and lower dipstick marks.

What is the mileage now on the oil? If you are in doubt, just change the oil and oil filter. Don't lose sleep over it.

For my money I don't think there has been enough real world experience using oil for 15K miles. My Mustang Owners Manual and "Intelligent Oil-Life Monitor" allow up to 10K miles, I'm changing it around 7500. I personally don't trust that the electronics in my car can detect every time all the conditions that could damage oil quality.

It should be okay. Those more expensive lubricants really aren't worth the cost in most cases.

15000 is a bit much. 10k might be better.

no oils last 12 months
you stick to 6 months or every 5,000miles and use regular oil as changing synthetic oil every 5k miles is very expensive

You can do that in a skillet on the kitchen stove. Try this; Use Mechanic's Engine flush to clean out the crankcase, change the oil and filter, then see if you get the same oil darkness.

You can't get that much time/distance between oil changes in a 15+ year old car, REGARDLESS of the oil used. You MAY be able to get more than the manufacturer's recommended interval with a newer oil, but you can't push an engine designed for oil changes every 3 months or 3,000 miles that far.