Oil Level

[CHRISTINE1]

i have a 2000RX300 with 112000 miles. it has only been serviced by a lexus dealer. i took it to the dealer in december for regular 110000 maintenance. in february i drove 1000 to florida. the engine made a noise on acceleration. i took it to a lexus dealer who said i just needed to use premium gas. that did not help. i returned to the dealer in florida who informed me that the car had only i quart of oil and an aftermarket filter. he found no leaks. the dealer put in 4 quarts of oil and told me to return after 500 miles. i have checked the oil level weekly and it has remained full.

what has driving all that time on one quart of oil done to the engine and is a dealer allowed to use an aftermarket filter.