Full synthetic is the only way to go.
I use to use Mobile 1 religiously, but was sold on AMSOIL after a friend who owns our local Carquest turned me on to it. After running it for a while and changing it every 3K miles in my Duramax, he kept telling me I was wasting money and that it could handle going a LOT longer than that. Since then I have personally run it almost 20K miles on one oil and filter change and I have customers who have run it much further using used oil analysis to guide them on their intervals. One customer (who later became a dealer) ran one oil change for over 34K miles without the oil viscosity or TBN breaking down and without any abnormal build up of dirt or wear particles in his 2008 LMM Duramax. A vehicle I might at that is known for fuel dilution issues.
That being said, don't buy into the "Change it every 3K and be done with it". Synthetic lubricant technology has vastly improved over the past 40 years, ever since AMSOIL FIRST offered Synthetic Lubricants to the automotive industry. They were the first to offer full synthetic oil for the automotive industry and have remained the industry leader ever since. No other oil manufacturer can legitimately make that claim. Their Signature series oils come with a WARRANTED recommended service interval of up to 25K miles or one year in normal operating conditions. Whether you run the oil that long or not, the peace of mind of knowing that the oil has the quality formulation to be ABLE to go that far while still providing second to none engine protection is worth every single penny.
Also, I have sold many oil changes to those that were comparing pricing between AMSOIL, Royal purple and others that chose AMSOIL because with the discounts I offer to forum members, they spent less while receiving what I honestly believe to be the best oil on the market. Keep in mind, unlike some other AMSOIL dealers, I became a dealer BECAUSE of my experience with the oil, NOT the other way around.
If I can help out with further information about AMSOIL products please don't hesitate to PM or
email me.