Savings and loan associations made fewer adjustable-rate loans in the first five months of this year than in the first six months of 1984, and tied the interest rates on a higher percentage of them to one-year Treasury bills, according to a study of ARMs released this week by the United Stages League of Savings Institutions.

Rates on the more volatile Treasury bills have been dropping faster in recent months than on other indexes, making them more attractive as a guide for increasing or decreasing the interest rates on adjustable-rate mortgages. The lender's cost of funds, as reported by the Federal Home Loan Bank Board or a district Home Loan Bank, was the most popular index last year.

ARMs' share of all mortgages dropped from 80 percent a year ago to 60 percent in May, as declining interest rates brought fixed-rate loans into range for more buyers, according to the study. The study also reported that the rate of ARMs delinquent for 60 days or more lagged slightly behind late payments on fixed-rate mortgages for January through May of this year, and that ARMs being issued now contain more protection for borrowers than a year ago.

The findings are the result of a survey the U.S. League of Savings Institutions made of ARMs issued by l,100 of its member institutions during the first five months of 1985. In its report this week, the league compares its findings with the results of a similar survey conducted in 1984.

League Chairman John B. Zellars said adjustable-rate loans have become a "permanent feature of the mortgage market," contrary to predictions by "some experts last year that ARMs would fade away" when interest rates dropped and home buyers switched back to fixed-rate loans. The majority of home buyers covered by the survey liked ARMs, which generally have initial interest rates about two percentage points lower than those for fixed-rate mortgages, Zellars said.

Many of the early ARMs lacked interest-rate caps to protect consumers against sudden, large jumps in loan payments. The potential dangers brought vigorous protests from Congress and some consumer groups, with calls for a ban on ARMs. Savings institutions say they need the adjustable mortgages, which provide protection against losses, if they must pay higher rates to attract funds.

The limits on rate increases may have helped hold ARM delinquencies lower than those of fixed-rate loans. The 1985 study showed that payments were late on 2.7 percent of all fixed-rate mortgages and 1.5 percent of ARMs in January, and on 1.9 percent of all fixed-rate mortgages and 1.3 percent of ARMs in May.

The figures reflect delinquencies on all ARMs being held by the savings institutions, according to Dennis J. Jacobe, senior vice president and director of research for the league. He said no data areavailable on mortgage delinquency rates according to years the loans were made and on ARMs foreclosures.

Limits on the size and frequency of interest-rate increases appear to be making ARMs safer for borrowers. ARMs with a lifetime cap on rates rose slightly from 93.5 percent of all ARMs covered in the survey in 1984 to 97.5 percent in 1985, and those with periodic caps on interest rates went up from 70.3 percent of those surveyed last year to 80.7 percent this year. Lifetime caps were based on the initial interest rate of the loan in 77.2 percent of all the mortgages surveyed, and nearly 20 percent set an absolute number for the lifetime limit.

Limits on payment increases were much less prevalent this year than last year, however, dropping from 59.1 percent of those covered in the 1984 survey to 39.9 percent this year.

All ARMs made in the Washington area in the first five months of 1985 had lifetime interest caps as well as limits of two percentage points or less on yearly increases.

The U.S. League survey reported that savings institutions have tightened loan requirements. Nearly 87 percent of all home buyers getting ARMs could spend no more than 28 percent of their gross income on mortgage payments, compared with 53.8 percent last year.

Mortgage banking companies, which make loans and sell them in the secondary market, issued far fewer ARMs than savings and loans, according to a recent survey of Mortgage Bankers Association members. Only about 20 to 25 percent of the loans they issued in recent months were ARMs, while about 60 percent were 30-year, fixed-rate loans, said Robert J. Spiller, president of the association and head of the Boston 5-Cent Savings Bank. The rest were 15-year fixed-rate loans.