Warren Buffett has made a lot of investments over the years, and many people follow the Oracle of Omaha's trades because of his proven track record of earning billions by playing the market.

That's why it pays to listen when Buffett makes a recommendation he believes is best for most investors -- especially if he also plans to invest 90% of his own estate in it

That investment is an S&P 500 index fund. And while some investors may have been unable to buy it before, it's now open to everyone. Here's why.  

Investor looking at charts on computer.

Image source: Getty Images.

You no longer need to be rich to buy this top Buffett recommendation

The S&P 500 is a major U.S. stock index that is widely viewed as a barometer of the overall stock market. Five hundred of the largest U.S. companies are included in the index, which is weighted by market capitalization so larger companies represent a greater portion of its value.

Buffett recommends it as an ideal investment for most people who aren't interested in (or skilled at) investing in individual stocks. And, unsurprisingly, his advice is sound because when you buy into this index, you're betting on large American businesses to succeed. Historically, that's been a very safe bet.

In fact, if you bought and held shares of an S&P fund at any time and held them for at least two decades, you would always have turned a profit. The S&P 500 has historically has provided average annual returns of around 10% going back to 1929. Most people can't beat that consistently -- although of course Buffett himself has proven it is possible. 

Unfortunately, while Buffett's recommendation may make sense for a lot of people, investing in the S&P 500 hasn't always been an option for everyone. That's because it can be expensive to get your money into an S&P fund. 

There are both mutual funds and exchange-traded funds that track the S&P's performance but mutual funds often have minimum investment requirements that put them out of the reach of beginning investors with small pocketbooks. And while ETFs trade like stocks, brokers traditionally required you to be able to buy at least one full share. That's a problem since index funds that track the S&P 500 -- such as the Vanguard S&P 500 ETF (VOO) or the SPDR S&P 500 ETF Trust (SPY) -- are currently priced above $300 for a single share.

The good news is there's no longer a need to buy a full share to invest in this Warren Buffett recommendation. Fractional shares are available from a growing number of brokers so you can buy a portion of a share based on the amount of money you have. Say you had just $10 to invest. You could use it to buy 0.027 of SPY, trading at $363.67 as of Nov. 27. 

While this may not sound like much, most brokers have also eliminated commissions so there's no downside to buying a small fraction of a share of an ETF. And you can benefit from the same percentage gain as investors with much larger pocketbooks. Plus, you can add to your investment over time as you get spare cash, all while the money you've already put into the market works for you.

Of course, you'll want to make sure investing in an S&P index fund is the right financial move based on your investing timeline, your investment goals, and your other investments. But if you want to buy into the investment Buffett believes is the best choice for most investors, fractional shares have opened up the door to do that even if you have very little cash available. You should seriously consider taking advantage of that opportunity.