I got a great deal on 5w30 Syntec ($1.66/quart) on closeout. Bought 12 cases (72 quarts) of it, thinking it would last for the rest of the life of my 1998 V6 Accord (120K miles) and be usable on most any other vehicle I would own down the road. Even though I didn't buy it from AZ, they were nice enough to swap 6 quarts for the 0w30 GC that they alone seem to carry. I currently don't drive more than 8K miles a year. OCI in owner's manual is 7.5K in normal conditions, but I do a change with the summer/winter seasons, so usually no more than 7 months and 5K miles on a fill. Car has a rear main seal leak, used to be to the tune of 1 quart every 2K miles on Mobil 1. I switched to Formula Shell (Costco had for $2/quart), thinking if I was losing oil, I didn't want to spend money on the synthetic stuff. That was 2 years ago. Now the seal has significantly diminished in leakage, though it's still present. I figured, since the Syntec is Group III, it shouldn't have any more of a predeliction to leak than the dino. My question is, since I was able to get some GC, would I be better served mixing in a quart of GC with the Syntec, or should I just stick with 100% of either when filling? Car holds 4.5 quarts or so, so it'd be like a 22%/78% mix of PAO and Group III.