I have just got a Nissan Skyline and the last owner ran it on e85. As not every service station sells e85 I now put e85 in every now and then and run 98-octane most of the time. The car is tuned to e85, so will it damage the motor to switch back to the 98, or stuff up the tune?
There are two ways of setting your car up to run on e85. One, the cheaper one, sees it tuned to run on e85 exclusively, in which case I don't believe it would actually run on 98-octane. The other, more expensive one is to fit a flex-fuel system, which is able to detect what fuel is being used and adjust the tuning accordingly to run on either fuel. The latter system requires the use of a sensor, the same used by GM in its flex-fuel cars, and an ECM to control it. That your car will run on 98-octane fuel leads me to believe that it could be fitted with a flex-fuel system. If it is there is no danger in running your car on either fuel. To check take it to a mechanic with experience in Skylines and they should be able to tell you what has been fitted to your car.