The index of leading indicators fell 0.6 percent in January, largely because of a drop in business orders for new plants and equipment, the Commerce Department reported yesterday.

Many analysts shrugged off the drop in the index, which often foreshadows changes in economic activity, even though it was the largest since April. "I think the decline is an isolated incident," said Allen Sinai, chief economist with Shearson Lehman Bros.

The decline in new orders was partly the result of a bunching of large orders for new commercial aircraft in December. In yesterday's report, Commerce revised upward from 0.9 percent to 1.5 percent the December gain in the leading indicators.

Over the last three months, the leading index has risen at a 4.3 percent annual rate.

"An occasional setback in the index is not unusual during expansion periods," Commerce Secretary Malcolm Baldrige said in a statement. "Even with this setback, the index rose at an annual rate of 5.7 percent during the past six months compared with a 2.4 percent rate during the previous half-year.

"This pickup points toward healthy economic growth during the near term," he said.

White House spokesman Larry Speakes noted that the index of coincident indicators, which generally reflects current economic conditions, rose 0.2 percent. "This indicates that steady growth currently is taking place, pointing to continuing growth through the end of the year," Speakes said. "Even though the leading indicators declined slightly, the Reagan recovery continues at a healthy pace."

Neither oil prices nor the value of the U.S. dollar is included directly in any of the indexes, although the decline of both is expected to give the economy a boost in coming months. "The favorable implications of their recent declines will be reflected indirectly in various components" of the leading index, Baldrige said.

Of the 12 indicators comprising the index, 5 improved, 5 worsened, one was unchanged and one, the change in inventories on hand and on order, is not yet available.

In addition to the drop in new orders for business-investment goods, a slower rate of increase in business and consumer credit and a drop in net new business formations contributed the most to the decline in the overall index.

The strongest positive effect on the index came from an increase in new orders received by manufacturers for consumer goods and materials and a decline in the level of initial claims for unemployment insurance benefits.

"Of the components that were down, one of them, real money stock, is likely to be up in future months," Sinai said. "Certainly in the coming months, the stock market component will push the index higher, and so will the building permit component," he added.

Sinai, like many other analysts, expects business investment in new plants and equipment to remain relatively weak in 1986, particularly as the oil and gas industry cuts back its investment plans as its income is squeezed by falling prices.

But with a lag, the same oil price declines should bolster consumer spending for other goods and services. Meanwhile, interest rates also have fallen, partly as a consequence of lower oil prices and diminishing expectations for future inflation. With a lag, that should encourage more investment outside the energy area, analysts said.

January's decline "is not indicative of an economy that will be weaker later in the year," Sinai said. "Indeed, from all indications, the economy later in the year will be very strong."