There are some stocks you just buy and hold forever.

These stocks represent businesses so fundamental to the daily lives of so many people, they deserve to be at the core of any serious long-term investor’s portfolio. Buy them, never sell them, let them ride for decades and reinvest the dividends along the way.

Some investors might be skeptical of such an approach. It seems uncharacteristic that a class of stocks could truly be held for such a long period of time, regardless of circumstance.

But that’s exactly what I’m saying. The very nature of a “forever hold” is that, during a 30-50 year holding period, nothing generally happens that these stocks can’t overcome. And there’s only one group of stocks that sell a product used so globally, that is so intrinsic to almost every person’s life, that is such a rank necessity for almost every business, that it could withstand the test of time and generate serious long-term gains for investors.

I’m talking of course, about oil stocks.

Companies like Exxon Mobil (NYSE: XOM), Chevron (NYSE: CVX), ConocoPhillps (NYSE: COP) and yes, even British Petroleum (NYSE: BP), are great long-term bets for investors. Smart investors know that in good times and bad, through wartime and peace, the world needs oil. It doesn’t matter what the price of oil is day-to-day or even year-to-year. As my colleague Andy Obermuller recently noted, the easy oil is gone, but these big oil players know how to find the rest. [Read: 5 Reasons Why Exxon Mobil is a Buy] If you hold on to your big oil stocks, you will eventually win.

Case in point: a friend of mine had a grandfather who recently passed away and left him a hefty inheritance. It consisted of a stock portfolio with purchases between 1955 and 1972. Even though grandfather had invested the same amount of money in a basket of different stocks, the oil stocks had grown to the point where they made up 53% of the portfolio.

Obviously, the grandfather never re-balanced his portfolio, but I think the point is clear.

Most of the time, I delve deep into a company’s financials before making an investment. I analyze it to the nth degree. I want to know everything about a business before I commit a penny to it. But oil stocks like these are no-brainers. A quick glance at their financials is just about all you need.

Big oil stocks generate hundreds of billions of dollars in revenue every year. Their annual net incomes are all in the mid-eleven figures. They push out $20 billion to $30 billion in free cash flow each year. And if you take the cash flow for any one of these companies, it would cover all of the long-term debt for all of the big oil companies combined.

I know all about the big push for green energy. I know all about Cash for Clunkers. I know all about how the world is allegedly coming to an end from global warming.

None of it matters. The world runs on oil. It will continue to run on oil for the foreseeable future.

In addition to selling what is practically the world’s most essential commodity, each of these stocks also offer steady, growing dividends. Conoco yields 4.0%, Chevron pays 3.7%, Exxon doles out 2.9%. These dividends provide, at a minimum, an inflation hedge while awaiting capital appreciation.

A quick word about BP. The company has wisely suspended its dividend in order to redeploy that capital into the Gulf spill cleanup efforts. The spill did not change the company’s business, however. The market took BP down, but I think we’re seeing that the company is going to be just fine in the long run, and that this is probably a great opportunity for significant capital gains.

Action To Take –> Do I really mean buy Exxon Mobil, Chevron, ConocoPhillps and even British Petroleum at any price, whether it be now or later? Well, if your time horizon truly is decades, then it shouldn’t matter. I think these stocks are, quite frankly, great core holdings for any long-term portfolio.

If you want to buy the most undervalued big oil stock relative to its peers on an enterprise value/EBTIDA basis, I’d look at Chevron. If you want a quick jump before settling in for long-term gains, buy BP. The point is, buy them, forget about them, and give your grandkids the gift of big profits.