There are a few engine mods that actually work, most take considerable time though.
the proven list of mods is here if you haven't previously come across it there is an engine/drive train section
65+ Vehicle modifications for better fuel economy - EcoModder.com
The wiki has data for only a few of these mods
Car Modifications Main - EcoModder
One user suggests Synthetic oil give no gain larger than 1% on MPG
Alternator delete can give up to 10% but there are obviously come consequences to this mod
In the unproven section of the wiki I have put a few in which I know without any doubt have worked for me.
Car idle reduction and cold start idle reduction but there are also some consequences to these mods as well.
Good luck