Objective information would be helpful: 1) consider performing a used oil analysis after 6000mi. on successive fills of conventional and synthetic, with perhaps an intermediate analysis at 3000mi (pull it from the dip stick tube). Consider using oils by the same (major) brand, perhaps Subaru’s (made by Idemitsu). Keep the use conditions (driving type, climate) similar, try to include the same number of cold and warm months. 2) To determine if the oil truly runs hot, measure its temperature after an hour of steady running on a warm day (use a surface thermometer or a non-contacting thermometer on the oil pan or stick a thermocouple down the dipstick tube) and compare to the recommended operating range.
For what it’s worth we have a 6cyl. 2006 3.0L Outback and compared Subaru brand conventional and synthetic and Mobile 1 (their regular “synthetic”) on two fast, hot cross country trips last summer - the conventional was changed at 5700mi., looked okay, but over the last several hundred miles it felt less lubricative between the fingers oil consumption increased (no make up needed). The synthetic went 6700mi. with consumption rate also accelerating near the end and required 1/2- 3/4 qt. of make-up. The standard grade of Mobile 1 performed similarly to the synthetic Subaru. Gas mileage records showed no differences between the oils.