Using 1 quart a mth. no visible leakage. i can tell when it starts getting low by how it runs. live in mx. 110 /87% humidity. is this normal usage? i never had to put in extra oil and get it changed every 3 mths thx

One quart per month is a bit more than I'd expect but you didn't mention how many miles you drive during that month.If you live where the ambient temps are very warm, it may help to use a good synthetic. Synthetic oils do not burn off or break down chemically as fast as conventional oils do. I'd use a 20w40 grade.
Depending on the miles you've driven you shouldn't have excessive ring wear but the valve stem seals may have dried out and cracked. If the usage remains constant and there is no smoking etc, you could either leave it alone or have new seals installed.
Average driving is approximately 1,000 miles per month so when figuring consumption, use that as a baseline. One quart per 1,000 miles in very hot conditions is a bit high....I'd expect to see 1/2 qt or slightly less. One odd thing though is that generally, especially in an engine that holds six quarts to begin with, at five quarts, the engine should not feel any different!!!
Also.....you use miles to determine oil change intervals. Oil is best changed at between 3K and 4K miles. Only way you use the 3month method is if you don't use it much. (you can run most good synthetics longer than that)