My neighbour works on cars a lot and he says the oil we were using is no use in a car thats done over 100,000 miles and has said that could be the reason we are losing oil, it being so thin.
I think its 15/40 he says would be best.
I showed him the Lexus manual and he pointed out that the recommended oil was for cold temperatures, not for ours.
He'll drain the oil and fill it up with 4.5 of the thicker stuff and see if that solves the problem.
Apparantly he's known some other people who had the same problem till they converted to the thicker oil.