Which oil i need?

[samn1968]

Hello :

i have a 96 2.5 outback with 199 Thousand mile i been using 10w 40 conventional oil, the car burns a little oil i want to switch over 20w50 conventional oil. will this change of oil weight will affect the engine performance?

 

Thanks

[ttnio]

performance no, gas mileage might.

 

a little.

[Brighton96]

That's REALLY asking a lot of an old tired oil pump, I would definitely go back to 10w-30 or 5w-30. And then I'd use either oil spec of diesels or turbos, both of which are "more durable" in regards to burning less under those conditions.

[doublechaz]

I've never seen diesel spec oil that wasn't 20w50, but I don't own one, so...

 

The oil pump is two gears. One is an inside gear, one is an outside gear. The outside has one more tooth and is offset to the side. As they mesh it pulls oil in one side of the cresecent shaped openning and pushes it out the other side. Pretty much nothing to go wrong with it or wear out unless you run the engine out of oil, have chunks of metal in your oil (at this point your pump is your least worry), or rev it to 15000 so the oil cavitates and pits the gear teeth.

 

If your pump *is* damaged then it would be bypassing oil instead of pumping it. Thicker oil will allow it to bypass less so that's a win

 

If your bearings are worn so that your oil pressure is a tiny bit low, thicker oil will help raise pressure in the loose bearings. If your bearings are a lot worn thicker oil may refuse to lubricate any remaining tight bearings as it flows through the loose bearings, so that is a minus.

 

Your rings may fail to remove as much oil from the cylinder bores if you run only 20w50 so you might burn more.

 

If you raise your oil pressure by running thicker oil the energy to make that pressure has to come from somewhere. Your HP/torq will go down. Maybe as much as 1 HP. That is where you can pick up a small drop in MPG.

 

All that said, I run (in 120 F ambient) 3 quarts of 20w50 and the rest 10w40 because I have slightly loose cam bearings. In the winter I run 1 or 2 quarts of 20w50 and the rest 10w40. That's on a 300,000 mile never been openned engine with about 150,000 of those miles a full throttle redline. It runs great and I drive it across the continent without hesitation. But I use a quart of oil about every 1500 miles. If I run all 10w40 I use maybe a pint in 3000.