Can you tell me about this Nordyne furnace?
The model # & serial # should be on the inside of the access panel, which you can use when you call Nordyne to find out what year & month it was made. I couldn't find anything on the web telling me how to decipher their numbering system.
While I don't know anything about Nordyne personally, if it is a 90+ as you believe, that tells me that it's 90% efficient, which is pretty good.
The only other recommendation I can make is to have regular servicing, which should keep it operating at peak efficiency.