My wife has a 2013 Optima still on original engine just over 130,000 miles now. I know there is carbon building up now over the pistons and oil is a dark as a sharpie pen I used full synthetic Mobil 1 for the past few oil changes wonder if anyone can recommend another brand for the motor or if I should stick with what I been using.
I would stick with the same oil. How long of an interval are you going between changes?
I recommend no more than 5k miles with full synthetic.
Been doing every 3,000
If you live in a place that gets really cold (I mean below -15°C / 5°F), I'd recommend to use 0W-XX oil for winter. I use 0W-30 oil for autumn/winter and 5W-30 for spring/summer on my 2013 Kia Sportage (it has the same engine in it as in Optima, but the volume could be different, mine is 2.0L). Currently using AmsOil, but there are many good oils out there — make sure to use ACEA A5/B5 spec oils that have been certified as API SN and up (SN Plus, SP).
As for the oil base — "real" Full Synthetic (base stock group IV, with PAO and Esthers in it) is better that more commonly available Full Synthetic oil, which is based on hydrocracked base stock group II/III/III+ (also known as HC-Synthetic or VHVI), and that is better than Semi-Synthetic or Mineral (Conventional) oil.
As a rule of thumb, change the HC/VHVI oils every 5,000 kilometers / 3,000 miles, and PAO oils every 8,000 kilometers / 5,000 miles. Oil change interval also depends on how you use your car: highway driving is much less stressful on the engine, so you can change the oil a little bit less often, while the start-stop traffic jam city driving is really heavy on the engine, so you have to change the oil more often.
Also, if your Optima has a 2.4L engine, I'd recommend you to check the oil pressure every now and then.
These G4KE engines had a fancy experimental high-performance oil pump installed in them, which consists of 2 smaller pumps that work on different engine speeds (one for idle and low RPM, another for high-speed and high RPM). These smaller pumps then connect to a control valve that switches between those two according to mode the engine is in at the moment. This valve can get stuck or broken and it'll either result in an insufficient oil flow or block the oil flow altogether, which then can lead to the engine getting jammed.
This is a common problem in 2.4L Optima, Sportage and Sorento vehicles found in eastern Europe, and most of the time it's caused by bad-quality motor oil or the fact that the owner didn't change the oil at required interval.
If this oil pump fails before the lack of oil jams your engine, you can go 2 ways fixing it: you can either replace it with an identical G4KE oil pump, OR you can install a much simpler-design (and much more reliable) oil pump found in 2.0L G4KD engines. It's much cheaper and it doesn't have this control valve problem, because it's a single medium-performance pump instead of 2 different pumps jammed into one casing.