Hi, I picked up a really clean GX470, and I'm using it as daily driver to work 3-4 times a week. Average about 1500 miles a month, probably 75%+ highway. Have all service records from previous owner. He did 5k changes with conventional...mostly at the dealer, but there are a couple of Midas specials thrown in here and there. It runs and drives phenomenally for an 04. Engine is not leaking or burning any oil. Really wanting to extend my OCI to 8-10k and wondering if it's safe to switch to a quality HM synthetic like M1 EP HM 5W-30. Or even the new RGT synthetic, but not sure I could extend my OCI with RGT, and little worried my MPG will drop slightly. Any recommendations?