Hey Used RX300,
I had the exact exact exact same problem. I bought my 2002 RX300 3 months ago. I changed EVERY fluid in the car. Then around 3000 miles a week before I was ready for an oil change, my oil light came on. I was down 4 quarts of oil!!! I looked at the oil cap and it looked caked on with burnt oil as was the top of the oil case. I assumed the previous owner let it run low and it turned to sludge since I've never seen such sludgey oil.
Just last night, I changed the oil myself but first added GUNK oil case cleaner with the engine running for 5 minutes as per directions. Then once the drain plug was open and all the dirty oil was out, I ran brake cleaner through to clean it up. I'm not recommending this necessarily, but I did it anyway. I then rinsed the brake cleaner out with fresh oil and allowed that into the bucket. Once everything was well rinsed with oil, I reinstalled the drain plug and put in 5 litres of fresh "high mileage" Valvoline. I'll be monitoring it closely.
Perhaps running a slightly thicker oil like a 10w40 instead of 5w30 might help. I might even try some compression treatments like STP, but normally I NEVER add any oil products since they often cause more trouble then they fix. I'll be watching this forum carefully
