1983 Nissan Skyline Pricing and Specs

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. 

They can be converted successfully using a vapour-injection system, or better still, a more up-to-date liquid injection system. Both will deliver top performance, but the liquid-injection system will deliver the best performance. Finding a kit is a little difficult, the demand for them isn't high, so no one to my knowledge has taken the trouble to develop one, but the guys at the Australian LPG Warehouse (03 9770 2287) can develop a kit for you and it won't cost you any extra.

THE R31 Skyline is robust and generally reliable once you've become used to the howling diff. I'd suggest you test for leaks from cylinder to cylinder and from each cylinder to the water jacket. It could be a head gasket leak.