'93 300d here, 198k miles,
Summer mileage is great, usually average 30-33 with most trips being my 8 mile urban route to work. On road trips where speed is constant around 50-70 all day it can reach high 30s. Broke 40 once, but that was a one time deal.
Winter is generally about 15-20% worse.
Fresh air filter helps a lot I've found. As does good oil and a quality oil filter. Also, in my experience, the combustion chambers like to leak which is bad for gas mileage. If you're buying for good gas mileage, have a compression check done before you buy or at least look for diesel goo around the injectors. Cost is about $150-200 per cylinder to have them resealed.