While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Marathon Petroleum Corp. (NYSE:MPC) gained slightly today after Imperial Capital upgraded the petroleum refiner from in line to outperform.

So what: Along with the upgrade, analyst Ann Kohler boosted her price target to $98 (from $76), representing about 12% worth of upside to yesterday's close. While contrarians might be turned off by the stock's strong rebound since early October, Kohler thinks there's more room to bounce given the attractive growth and margin-increasing opportunities that Marathon still has ahead.

Now what: Imperial now expects Marathon to post 2014 EPS of $8.61, up significantly from its prior view of $6.25. "Due to its integrated operations, we believe MPC remains well positioned to balance growth capital investment across its system while continuing to return cash to its shareholders," noted Kohler. "While MPC looks to shift capital toward Speedway and its midstream operations, businesses which have consistently provided more stable cash flow stream and higher valuations relative to refining, it continues to evaluate refining projects focused on margin enhancement opportunities." Given Marathon's hefty debt load and highly volatile shares, however, I'd wait for some of the exuberance to fade before buying into those prospects.