I'm changing the engine oil soon and can't decide between two oil choices, my biker friends all say Motul 15w50 7100 fully synthetic and the Rocket 111 manual says Mobil 4T 15w50 fully synthetic, which one should I go for?
Every owner who coverts their bike(s) wants to keep them in tip top condition but it's easy to get hung up over oil choice and over think the issue. I used to be the same, trawling through reams of info and tests/reviews before deciding what to buy. Nowadays I just go with the makers recommended oil specs, then shop around for the best deal. Could be Castrol, Shell, Silkolene etc..etc. Currently it's Shell in the Z and the Scrambler because They were the right price at the right time. The Berg uses 15/50 Elf FS as it's the right spec and good price. Years ago I bought a new FZR1000 and Yamaha recommended Motul. I asked a guy on the Yamaha stand at NEC bikeshow that year who said they recommended Motul as they sponsored their race team. In answer to your question, both are premium brands so I reckon your pistons are safe with either.
I tend to use Shell Advance Ultra 10W-40 4T Motorcycle Engine Oil Fully Synthetic same spec and so much cheaper about £25 for 4 litres delivered off eBay,
Mobil is a premium brand and I have found some applications (performance V8) were it made a noticeable difference against other brands, but I find in the fully synthetic it is the spec that counts. My Triumph dealer used Motul.
Follow the recommendations from the people that built the bike if possible. Not saying the others will ruin it, but if you can afford it....stick whatever the manual states. The recommendation is there for a purpose.
Just check the API rating on the cans you will likely find they are the same that's all you really need to check other than the viscosity.