Merc 25W40 oil

[oscar7]

I have 1992 merc 5.7L inboards. Have always used staight 40 wt oil. Dealer convinced me to switch to merc25W40 oil. Oil pressure is now 45psi at idle, and pegs the meter when I rev the engine in neutral.<br /><br />Merc says the oil should be fine, but that I probably have too much oil in them. Manual says it should take 5 1/2 quarts of oil, but to get a full reading on the dipstick it takes about 7 quarts. Does merc make different size oilpans on these? The previous owner mentioned something about a deep pan, and he always used about 6 1/2 to 7 quarts to fill. <br /><br />Will likely go back to straight 40 weight cause I didn't have this problem using it. But the question still remains: Do you fill to full mark on dipstick, or what it says in the manual?

 

[Don S]

Re: Merc 25W40 oil

A previous owner may have changed the pan, without actually being able to see the pan, I would suggest you stick with what the dipstick says. Normally a 5.7 with 1 1/2 to 2 quarts overfilled will have the oil areated by the crankshaft and cause lifter noise and fluctuating oil pressure. Your high oil pressure sounds like a seperate problem. Changing weights of oil shouldn't cause the high oil pressure that you see. You might try another oil filter before you go back to the 40W, it could be defective. If you still have the high oil pressure showing after changing the filter, put on a mechanical gauge to verify the oil pressure is actually that high and you don't have a sender/gauge problem.

 

[oscar7]

Re: Merc 25W40 oil

Don, thanks for the response. All was fine before I changed oil and filters. It is happening on both engines which makes me think it is the 25W40. Suppose it could be the filters. But do you think it likely that I would get two defective filters?<br /><br />I agree, I wasn't expecting the difference in the oil to make that change in oil pressure.

 

[seapack]

Re: Merc 25W40 oil

Oscar,<br /><br />I recently rebuilt my 5.7 Mercruiser inboards. The oil pressure at idle is about 30-40psi. When running over 1500 rpm the needles bury at 80psi. New oil pumps were put in during the rebuild. I run 25W-40 Mercruiser synthetic blend. This oil pressure is normal. The straight weight 30W used to break in the engines would run 10-15lbs lower pressure. I would stick with the 25W40. There is nothing wrong with the higher oil pressure.