Going from memory of the other thread.....this is the first major highway trip since your mechanic found and (hopefully) fixed a carburetor issue.
My 2 cents:
* Change one thing at a time so you know what's what. Let this be the trip to evaluate the carb thing.
* Oil isn't going to change MPG by a measurable amount. If it needs changed, change it with whatever you're running now....just to keep it a constant.
* Unless there's solid evidence to suggest at HEI problem, leave that alone for this trip.
When it comes to mechanical things....I am very prone to throwing "stuff" at the wall just to see what sticks. It's an urge I am slowly learning to resist. Most recent example for me is my backup chainsaw that suddenly stopped running. Trying to methodically test and eliminate theories about compression, spark, and fuel.
