Hello, I recently purchased my 2006 GMC Envoy 4.2L I-6 about a year ago (used obviously). At that time, it had ~129,000km on it and I had an oil change done at the GM dealer. I want to start doing my own work on the car, and I've been getting some really conflicting advice about oil changes: specifically, what OCI and which oil I should be using. I called up my dealer and one person started telling me that I need Dexos 1 oil and I was like [censored] is that I don't think I've heard of it before... Long story short, I found that it's not even required for vehicles older than 2011. My service rep told me that you should just go by the GM Oil Life Monitor (when it gets to 10%) or every year if you don't get to 10% in a year - that seemed reasonable. Moving on to oil - the owner's manual says 5W-30 and my the car has been serviced at the dealer from when it was new until now (so conventional oil is my guess). Right now, I'm seeing Castrol GTX 5W-30 at CAD $54.99 / 12L (CAD $4.58 / L or USD $3.33 / qt) at Costco but I dropped by Canadian Tire to pick up a filter and noticed that Penzoil Platinum was on sale for CAD $31 / 5L (CAD $6.2 / L or USD $4.5 / qt). The dealer people were adamant that I should never switch from conventional to synthetic with my mileage... so I held off on making a decision. Is that something I need to be worried about? Should I consider moving to synthetic if the price is comparable? Sometimes, clearance synthetics are within $1-2 / L of conventional so that seems pretty good. If I do make the move, and given my mileage, do I need to stick with synthetic from then on (or is that a myth)? From what I can tell, the car has been maintained very well (it was a company car, so it was driven a lot - but maintenance was always done regularly and at the dealer). I checked the oil about a month ago and it was sitting happy in between the markings on the dipstick. Looked pretty black however... Any advice? I'm new to car ownership, but I'm pretty handy and want to learn how to do these things on my own. Figured I'd make a post on here and have some informed people try to educate me! Thanks!