Last two UOA's done on my wifes Maxima have sheared down to 20wt within 3k. Is this a given or are there xw-30's (OTC dino)out there that will stay in grade for 3k?? I've noticed the "High Mileage" oils start at a higher 30wt,is this the best option?? By the way, the oils mentioned above were Tropartic 10w-30 and Castrol 5w-30.Any ideas??