Originally Posted By: 4wheeldog
As far as I know, there are 2 possible sources of "Rattle" in Subaru 2.5 motors.......Piston slap and rod knock.
Piston slap is loudest on cold fireup, and tends to quiet down when the engine reaches operating temperature. It is not a "Death rattle" as many engines live for 100k or much more, with this sound effect in place, with no other consequences.
Rod knock (Or even main bearing knock) is a whole different problem. It is generally quietest at cold startup, and gets worse as the oil gets up to operating temp. That is a true death rattle, and will only get worse in relatively short order.
Any other knock is probably an external problem. The valvetrain may click or tap, but nothing that could be described as a rattle. Belt tensioners are an external source of knock, and will cause the car to come to an unscheduled stop soon. A timing belt tensioner (Or idler) will fail with valves meeting pistons.
As far as the first 2, (Piston slap and bearing knock) I do not believe oil choice is going to have much effect, though 40 or 50 weight oil may give some (Probably short term) relief.
Right.
If it's piston slap that's most noticeable on start-up the oil choice isn't going the make much difference.
If on the other hand you have real "death rattle" which is rod knock at normal operating temp's, heavier oil can forestall the inevitable.
