As for the K&N air filter that I'm running on my 87 SDL, I intalled the filter just after I purchased the car. So I don't have any before and after data on this car.
My 94 Chevy Suburban (k1500 1/2 ton 4 wheel) always got about 13.3 on every road trip, and about 12.5 mpg in town. After installing the K&N with no other changes or repairs, the mileage jumped to just over 17mpg on the highway. The throttle response in getting the tank rolling was also much better.
Being that the $43 filter paid for it self in one 42 gallon tank of fuel in the suburban, I installed one in the 88 Saab 900 turbo spg. The Saab went from 30mpg highway to 33mpg.
With good luck from the K&N filters in the two other cars I installed one in the 300SDL right after I purchased it.