Ron,
I thought you got a 49mpg
before I read the whole story.
You want an Instant 40mpg cheap beater?
Get something like year 93 1.5 Honda Civic with 5 spd. My friend has one. I can't believe he constantly gets about 40mpg crusing on the hilly I-68 to Jersey. He has a 4dr sdn. If you want to beat him, get a 2dr hatchback from the same version of engine and gearbox.
You may miss the power from your 240sx after that.
My personal experiences on my cars is, everytime after I do my oil changes, for the first or second tank of gas, I always get 2 or 3 miles better per gallon. After third or fourth tank, it will get to the usual...maybe the oil gets dirtier...
