I just bought a 2015 Outback Limited. Will the gas octane level i choose make a difference in the performance and life of the engine?

Tom is correct, use what is recommended in the owners manual. I believe that both the Four cylinder 2.5 and Six cylinder 3.6 engines use the same octane. Do not use a lower octane than 87, which should be the magic number here. In the Midwest, on highway 70, I have noticed a few fueling stations offering 85 octane and some using ethanol. If you use 85, you may experience some pinging sounds which is NOT GOOD. You wouldn't put recycled oil in your car, make sure that you buy gasoline from a reliable source and use the proper octane. If you're on the road you can trust Pilot stations, just be sure to get 87.

Carole, if you're concerned about the life of the engine, follow these simple steps, 1. follow the maintenance schedule, 2. don't race the engine or drive it hard, and 3. when taking off from a dead stop, go easy for the first 5 to 10 mph, then, do whatever you want. This will ensure that the fluid in the transmission is fully flowing in your systems. Enjoy your car. No need for any fuel additives, not recommended. And, use a good synthetic oil, Mobil One is great.