Have OnX since 2015, did the 1.5 - 2 during 18 months, it works just fine with the OnX, now, you have to take daily 81mg ASA with it, I dropped because the 12 doctors in my close family, sister, cousins , uncles, all advised me to STOP the Aspiring together with Warfarin, so i did, and moved to 2 - 3 NO aspirin. There are studies that show 1.8 - 2.8 works just fine for all modern Bileaf valves; IF you test Weekly. Other than that, i know off a complete Cardiac Institution where all mech valves are managed at 2 -3 in Aortic position for past 35 years, no problems there; IF you test weekly and want to do 1.8 - 2.8 for an aortic valve that works; Aspirin+OnX+1.5-2, worked for me, but you are always worried if drops below 1.5. And as per OnX, it is Unique in the sense that is the ONLY mech valve that opens 90 degrees, St Jude does not, and that is why many Surgeons today, ONLY use OnX