When I bought my xc90, it already had 135,000 miles on. Unsure of what oil it used, on my first oil change I used castrol syntec 5w30. All was good until temps dropped and I realized it was way too thin of oil and there was a good amount of chatter from the engine. I downgraded to castrol hi mileage, and have been using it since then. I have put another 120,000 miles on it and its run pretty well on the hi mileage oil. Engine is still quiet and smooth to this date. I do occasionally top up when the oil level is low with conventional oil which hasn't caused any issues.
I recently found out that castrol gtx is a synthetic blend, which is same as castrol hi mileage. Of course hi mileage stuff has extra additives.
So, if the oil is changed frequently, is conventional castrol gtx oil ok to use, considering there has been a fair amount of improvement in the formulations of conventional oils these days? I always hear of people putting whatever is cheapest and changing it frequently and their engines last upwards of 350k miles on original engines.