Last year, I switched from the Merc 25W40 oil to Amsoil 15W/40 Heavy Duty Diesel & Marine oil. With the Merc oil, the pressure was just over 40psi at all times. With the Amsoil, it is the same except at dead idle once the oil is hot. It then drops to around 20 psi. The pressure picks back up when I bring the RPM"s up even slightly over idle. Someone told me a while ago that this okay and that the engine is still getting adequate oil, but the pressure is lower because the Amsoil flows easier. I guess I'm looking for a second opinion.

If this is a problem, I may switch to Royal Purple's straight 40-weight oil. It seems to be the only detergent syn oil out there available in straight weight. Redline's is non-detergent.

(Yes, I know, I saw Bob Teague's comments in Powerboat this month. I still don't buy the idea that synthetics don't work better in marine engines.)

I had basically the same problem as you described. My guage would fluctuate what I would consider to be excessive. I temporarily mounted a mechanical guage and found the pressure was actually staying steady. The problem was in the guage or sending unit. I switched the stock Mecrcruiser sending unit for a Gaffrig unit-----problem solved..

20 PSI at idle is no problem. If you had 40 PSI at WOT before the switch and you still have 40 PSI except at idle when the engine is warm there is no problem that wasn't there before, if at all. I have seen engines with only 5 to 10 PSI at idle with 35 PSI running and they to were OK, or maybe a little worn. Excessive oil pressure will not help performance or bearing life. Watch your oil temps and if your 40 PSI turns into 35 and then 30, you may have some probs. Just my 02.

don't confuse pressure and flow. if you would like to know more about royal purple products??...please contact me. ...i can bring it to the st.cloud race for you and save ya shipping cost! my cell is 734 635 1697.

[QUOTE]Originally posted by audacity
[B]don't confuse pressure and flow. QUOTE]

Meaning what ?? I thought I was properly distinguishing between pressure and flow.

Ric,
Did you notice any more
oil consuption after you switched to
amsoil ??
I have some of the same stuff (15w40 amsoil)
that I'm waiting to try this spring.

I too, saw the Teague article,
but after the thread discussing
synthetic oils a couple of months back,
I'm going to try it.

jt

I have always run Royal Purple 40 weight synthetic,,, been very happy with it,, had over 600 hours on the motor with no problems.

do you know the flow rate of a given oil at a given tmp in your system?...you never mentioned: what your tmp vs psi are. or at what engine speed...from here we can make educated guesses based on oil manufacture specs (engine specs too)...some oil companies do not give out this info...

with what oil? what is your oil tmp, psi, rpms at extended wot?...then what is it when you drop right off plane?

actually amsoil does not give you this info...they just tell you its great...a flash point and a freezing point and some other #'s we are not too concerned with.