My family inherited a rather mistreated 90K Toyota Tercel. It had lots of dark sludge in the oil filler cap and black sediment speckles on the dip stick. It also had an annoying oil leak from the area of the crankshaft to flywheel seal. I gave it Mobile 1 10W 30 for a few thousand miles then Mobile 1 5W 30. Curiously the oil leaked stopped completely after the change to 5W 30. This seams counter intuitive. Does anyone think that there is a connection or is it a coincidence?