I am using Amsoil full synthetic 0w20 oil. No issues.
Looks like Amsoil now has three different 0W-20 "synthetics":
- SAE 0W-20 Signature Series 100% Synthetic Motor Oil ($10.55 quart)
- SAE 0W-20 XL Extended Life Synthetic Motor Oil ($7.65 quart)
- SAE 0W-20 OE Synthetic Motor Oil ($6.10 quart)
All prices per their website, however, discounts may be available, although I assume shipping charges may apply (not sure).
http://www.amsoil.com/a/synthetic-motor-oil-engine-oil
The Signature Series 100% Synthetic Motor Oil is a high quality product that contains large amounts of true PAO/Ester synthetic stock characteristic of a premium synthetics like Mobil 1, Castrol Edge, Pennzoil Ultra, etc. The XL Extended Life Synthetic is maybe about as good as other Group III synthetics such as Castrol Syntec, Valvoline SynPower, or Pennzoil Platinum. Not sure about the Amsoil OE motor oil, which I had not seen previously last time I looked on their website a few years ago (maybe it is a synthetic blend).
Strange at it may seem, the premium Signature Series, although theoretically a very high quality synthetic, is not API Certified, partly because it does not meet API energy saving specs. Also, some non-API certified synthetics contain too much phosphorus in the form of the additive ZDDP (Zinc Dialkyl Dithiophosphates). The API has limited the amount of phosphorus because phosphorus shortens the life of the catalytic converter. That is not to say that Amsoil Signature Series is anything but a very robust synthetic oil, but I would hate to get into an argument with Hyundai regarding a warranty repair issue if they found out.
Amsoil is not very clear about API certification for Signature Series on their website. They say that the oil can be used when API certification xyz, etc is required, but they don't explicitly say that the oil is API certified and don't display the API certification symbol on the oil container for the Signature Series.
If Hyundai allowed 15,000 mile oil change intervals to maintain their warranty, and if I was so inclined to only change my oil only that often, Amsoil Signature Series might be a good choice, but since that is not the case, I don't see the point. Also, I would not want to take even a 2% hit on MPG for no good reason.
From Wikipedia:
AMSOIL markets their products through a Multi-Level Marketing (MLM) network of dealers who sell to defined customer types known as wholesale customers, retail accounts and commercial accounts. AMSOIL also wholly owns and manages an MLM subsidiary called ALTRUM, which markets nutritional supplements.