You have some very good advice here already.

No air filter will increase MPG over ANY OTHER NEW FILTER. Why? All '82 Volvos have a Lambda sensor (Oxygen sensor). The sole purpose of this is to tell the EEC what the current air/fuel ratio (AFR) is. Now admittedly these sensors have a narrow operational range, but the EEC is pretty good at keeping the AFR at the designed level anyway, the Lambda is really for verification.

In order to get better economy (all things being equal), you need to inject LESS fuel for a given amount of air. That would be a "leaner" mixture. As soon as the folks at "Joe Blow" air filters tell you their filter will improve economy, you have to ask yourself you it changes the EEC or any of it's AFR inputs to accomplish this. Basically it cannot.

Sure, on a carbed car with a really bad filter design, you actually do see an improvement. That applies to basically no one in today's world.

The M/T is NOT the trans you want for a performance car. They are significantly weaker than the available A/T. Most of the "Turbo Brick" folks switched to A/Ts for this very reason. OR they spent big $$ upgrading to a stronger M/T like from a Mustang etc.
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JohnG 1989 245 MT @ 235,000