How often to change oil?

[Flavor 987S]

Every year, prior to winter hibernation:

987S Every 5-7,000 miles. First 4 years used Red Line 5W40. Now running Mobil 5W50 (hard to find oil). Blackstone UOA's are excellent.

993TT Every 5,000 miles. Running Mobil 1 V-Twin 20W50 (yes, a motorcycle oil). UOA's are also excellent.

*Chevron Techron Fuel System Cleaner prior to oil change.