Correct oil pressure?

[Oddy]

Hi all,
I'm new to the owners form and a first time tige owner, hope someone can help!

My 2008 Z1 with 5.7 MP seems to have oil pressure issues! When I start her up the pressure sits on about 40 PSI after about 30 minutes of towing (fully loaded) the pressure at idle drops to between 5-7 PSI and the alarm sounds for about a minute, if I touch the throttle it jumps to 20 PSI and the alarm stops, at towing speed the pressure sits on 45-55 PSI.
I know it's not a sender issue because I hooked up a mechanical gauge and it confirms the dash gauge.

Is this an issue? Or am I being paranoid? The owners manual says 5 PSI is not the best but still acceptable at idle!
I am going to try run a slightly thicker oil to see if it helps keep the pressure up, are there other things that could trigger the alarm? I have read on here that the transmission oil could be getting hot and also causing the alarm!

Any suggestions would be greatly appreciated.
Thanks.

[chpthril]

Welcome to the site.

Have you checked the engine oil level? What oil weight are you currently running? How many hours since the last service?

The transmission oil pressure will not effect the engine oil pressure.

[Oddy]

I serviced it at the time I bought it, it had a full service history but I think it had done about 70 hours before I bought it and serviced it.
I replaced the oil with the recommended SAE40 as the service manual says. I have only done 8 hrs since then and I carry on having this alarm and low pressure issue.
I noticed that on start up with SAE40 the oil pressure sits on 42PSI I have just done another service today and replaced the oil with SAE50 just to see what it does, the start up pressure is now 49PSI.
Is this bad? It drops about 5 PSI when it's starts warming up, there is no clicking noise up top, so I am guessing the oil is pumping up there!
The engine has 550 hours on it and I think its had a hard life, it's a salt water boat and it was loaded for a serious wake.

Do you know what else could be causing the alarm to go off, I did notice the depth gauge fluctuating drastically at one stage, does this model have a shallow water alarm?
Thanks.

[chpthril]

Ok, so you've a little over 8 hours on the boat and the oil was changed right after you purchased it. Have you checked the oil level with the boat sitting level in the water? Low oil level can cause low pressure.

If you have the service history, how many hours on average, between oil changes?

If you engine has the dual oil pressure senders on the brass fitting, did you pull that brass fitting from the block and test the pressure with your mechanical gauge? Its not unheard of for that fitting to get gummed up. This effects the pressure the senders are seeing, but not what the engine is producing.