I have a 2012 Sonata 2.0T and the manual recommends API Service SM, either 5W-30 or 5W-40 -- it says nothing about synthetic or otherwise. The dealer used Penzoil 10W20 (manual footnote: "for better fuel economy, it is recommended to use the engine oil of a viscosity grade SAE 5W-20 API SM"), and it was just replaced with Castrol GTX 5W30 (it says 5W30 on the oil cap). It appears to me if the oil meets the "SM" spec, then it is good. Here's what I've discovered on the internet...
http://www.castrol.com/castrol/genericarticle.do?categoryId=82915464&contentId=6003410
CASTROL GTX SAE 5W-30:is a premium, super multi-grade that provides maximum cold weather engine protection and can help extend engine life. SAE 5W-30 exceeds
API Service SM, SL, SJ and SH, as well as exceeding all the requirements of ILSAC GF-4/GF-3/GF-2 for PI Certified Gasoline Engine Oils and meets Energy Conserving Standards.
http://shop.advanceautoparts.com/webapp/...nzoil_8040017-p
Pennzoil 5W20 Conventional Motor Oil with Active Cleansing Agents continuously helps prevent dirt and contaminants from turning into performance-robbing deposits, helping keep your engine clean to maintain responsiveness. It meets or exceeds the engine protection required by ILSAC GF-4 (5W20, 5W30, 10W30) or
API SM and is specially formulated to provide extra protection against the harmful effects of stop-and-go driving and high-low temperature engine operation.
http://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/Switching_Synthetic_Motor_Oil.aspx
Ask Mobil
Question:
What Will Happen Switching From Synthetic Motor Oil to Conventional Oil? I have been in the mechanic business for 8 years and was always taught that you cannot go back to conventional oil from synthetic oil. I have seen the damage caused by going back to conventional. Is it true that you can switch back to conventional with no damage to your engine?
-- Jimi Peranteu, Essington, PA
Answer:
Synthetic oils will provide better protection than conventional oils, but switching back and forth between full synthetic and conventional oil will not damage the engine. Of course, this depends on the current engine condition and the quality of the conventional oil being used.
http://www.popularmechanics.com/cars/how-to/products/4213451POPULAR MECHANICS:
Q: After a recent trip to two major auto parts dealers for oil (5W30), I realized synthetic oil is slowly replacing [conventional] oil on the display shelves. The problem is, I'm getting conflicting information about synthetic oil….
A: Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swell characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad. The crank seals had become worn, in their turgid state, and then relaxed. The valve cover seals were compressed when swelled, and when the different oil was added, everything leaked like, well, an old English sports car.
Fortunately, the situation has improved; you should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print -- a lot of the "synthetics" on the market are blends containing a substantial proportion of mineral oil.