Can I use a higher octane fuel in my 400 4x4? I use 87 now.

 

You most certainly can, but it's not likely to give you much benefit. I had a 375/400 cat that would seem to ping a little more when I used 87, 89 seemed to cure it. But as far as power is concerned you probably won't notice anything different.

 

Higher Octane does nothing for lower compression engines. There is no power increase but it does burn hotter. Octane is to stop pre-detenation which can cause problems and damage to a high compression engine.

Always stick to the fuel and recommended lubricants as specified by the manufacturer.