(A digest of investment opinion from the world's leading financial advisers)

"Over time, the sum of the Dow's price-earnings ratio and the long-term average inflation rate has gravitated toward 18," notes Personal Finance (1101 King St., Alexandria, VA 22314). "There have been no exceptions in 50 years. Indeed, history shows that after the sum falls below 18, it rebounds well above that level. Currently, the sum is 14 - nearly a 50-year low. The implications are enormous and suggest a Dow move to well over 3,000."- If the Dow does make another run at 3,000, small-capitalization stocks, which were making a comeback after six years of underperformance before Iraq invaded Kuwait, could resume their winning ways. Putnam OTC Emerging Fund has made small caps pay to the tune of 17.9 percent average gains over the past five years by buying cheap companies with low valuations and sustainable growth rates and sticking with them. Recent favorites: Altera, Apogee Enterprises, Cellular Communications, CII Financial, Cirrus Logic, Harding Associates, Mentor, A Schul-man, Systems Software Associates, Watts Industries.

- "Mounting international concern over global warming could provide steady profits for investors throughout the '90s," says Jack Wynn, writing in Bull & Bear (2845 Tech Drive, Orlando, FL 32817). Wynn mentions four large publicly traded companies actively engaged in countering global warming: Atlantic Richfield, GE, IBM and Westinghouse. But he also recommends two little-known OTC companies with interesting technology in the area: Chronar Corp. and Peripheral Systems.

- "In an increasingly complex political and economic environment, companies with strong balance sheets will weather any storm," says Great Lakes Review (1331 Euclid Ave., Cleveland, OH 44115). In a recent report titled "The Joys of Debtlessness," Great Lakes Review recommended seven stocks with strong fundamentals, below-market price-earnings ratios, returns on equity above 18 percent, growing earnings and virtually debt-free balance sheets: Calgon Carbon, Federal Signal, Gibson Greetings, Lawson Products, Sanford Corp., A Schulman and Tech/Ops Lan-dauer.

- Gold funds are too volatile right now for conservative investors, says Donoghue's Moneyletter (P.O. Box 6640, Holliston, MA 01746). "But active and venturesome investors should allocate 10 percent of their portfolios to gold funds as long as inflationary pressures, political uncertainty, a weakening economy and a sagging dollar are combining to cloud the financial horizon." Donoghue's two favorite gold funds: Lexington Goldfund and Vanguard Special Gold.

- The biggest fallacy concerning junk bonds is that anybody who could be scared out of them already has been, says Derrick Niederman writing in Investment Vision (82 Devonshire St., Boston, MA 02109). "That's wishful thinking. Truth is, the junk market still teems with motivated sellers. The S&Ls, for example, are now obligated to divest billions in junk bond holdings by 1994, as prescribed in the thrift bailout program. And a nationwide limit on the junk holdings of insurance companies could be legislated in the near future."

- If you're considering a home equity loan, keep an eye on banks' processing fees. They're creeping up according to a study by the Consumer Bankers Association. Lenders now charge an average $23 for recording fees, $150 for appraisals, $84 for a title search and $228 for title insurance. Title insurance is required by 57 percent of open-end (revolving credit) lenders and 54 percent of closed-end (second mortgage) lenders.

(Investor's Notebook reflects the opinions of professionals. It does not endorse specific investments, and no endorsement is implied or should be inferred. For more information, contact the individual firms cited.)