Lexus RX300 with 95000 miles uses a quart of oil every 400 miles

no,there is an oil leak

we check oil every other fuel fill up. on old cars, every fill up
did you run car 10,000 miles to a oil change? unstated.
1qt/3000 miles is perfectly OK.
so 10000 mles is 3.3 quarts burned and off the top of the dip stick
if you drive fast, it burns more, got a heavy foot, or load in back>?
if you had done it right, you'd have already learned YOUR engines
consumption rates, and learned to check when IT , needs checking
we do this on all engines. all machines.
all engines burn oil, the exception , a church car, new, and never driven, far, or fast or for 10000 miles to an oil change.

your mech, was right, he was thinking , you dont check oil?

I drive quite a few miles in my 1998 LEXUS LS40 and have religiously changed the oil every 5,000 miles (and rotated the tires). Im pushing 185,000 miles on it now and I am due to change the timing belt a second time next weekend. That being said, I have never had any issues with the engine changing the oil at a 5,000 mile interval. I use the standard synthetic blend oil at the local quick stop type oil change places.

Use 4 quarts of a little heavier oil and 1 quart of lucas oil stabilizer that willslow leaks dramatically.It is not a fix but it will buy you time

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)

First dealer was NOT doing you any justice. Next oil change run a can of GUNK engine flush let idle 5-10 mins then change Oil & Filter and use a good oil like Castrol or Valvoline 10w-30. You won't have "supposed" sludge build up.

anywhere from 4-6 quarts. put 4 in and check your oil then if its still low add one quart and repeat process.

if you change oil every 2,500 miles , maybe is a oil pump failure, if not , then remove engine oil pan and clean oil pump strainer, use good quality oil

Check out this thread and take your Caddy for a couple of Wide Open Throttle runs on the Interstate. Clears out the carbon buildup on the rings that is causing oil blowby.

http://townhall-talk.edmunds.com/direct/view/.f1078f6/0