I would not recommend pressure canning apple butter.  The processing time for apple butter is only 5 minutes in a boiling water canner so your idea of using apple sauce time of 5# for 8 minutes using a pressure canner would be very over processed.  It would certainly not be a safety issue, but I am pretty sure the quality would be compromised.  My thought is that it would be very dark because of the high heat, the sugar could caramelize.  

My suggestion is to use your pressure canner as a water bath canner.  Fill that canner with enough water so that you have 1 inch of water over the top of the jars.  Be sure to put a rack on the bottom.  Heat water to approximately 180 degrees F.  Put in the hot apple butter jars, put lid on loosely with no weight and bring water to a boil.    As soon as the water boils, start counting the 5 minute processing time.  You can reduce the heat slightly to maintain a gentle boil throughout the processing time.  At end of 5 minutes, remove canner from heat and let set 5 minutes before removing jars to equalize the temperature and assure a better seal.  

Hope that helps.  Let me know if you have further questions.  Thanks for using Ask Extension

Nellie Oehler