Outside of the factory fill, I've used nothing but Mobil oils in my 2004 WRX. That includes Mobil Super 10W-30 from the first oil change to Mobil 1 5W-30 or 0W-40 since. My other car I'd pretty much only used Mobil oils, although the first change used Castrol Syntec 10W-30 and there was a time when I freaked out about ring seating and used Pennzoil yellow bottle 5W-30 for about 10,000 miles before going back to Mobil 1. I used Mobil 1 0W-30, 5W-30, and 10W-30 on that car, but after a while I switched to Mobil high mileage 5W-30.
When I've changed oil for my parents' vehicles I've been less brand loyal. On my dad's Buick Regal I tried almost everything I could find, including TropArctic (from Dollar Tree), Kendall GT-1, Chevron, Havoline, Shell, Mobil, Pennzoil (yellow), Castrol GTX, Valvoline, etc. That was back when a typical quart was $1.50 and coupon/sale/rebate prices could be under $1 a quart. Nothing quite like an oil change for less than $7 in materials (if I could find an AC PF47 for $2). The oil drain plug also had a rubber gasket, so I didn't even need to spend on that.