i have a 1999 gs400 but i believe it is the same. look under the front of the car and there is a cover with about 10-12 bolts covering the engine. remove the bolts and you will find the oil filter on the bottom of the engine near the front right tire. hope that will help you.
I too, have a 1999 Lexus GS400 and there are about 16 bolts to that cover underneath and I found that there is no need to remove that cover. (PAIN!) You can use oil filters: FRAM PH3614, Havoline T-29, Pennzoil PZ-21, Quaker State QS3614, Valvoline V025. The pan bolt is 14mm. Engine takes 5.5 quarts of oil. (Mrs. Arias☺)