Choice of oils is really up to you. Marine technology was years behind automotive at that time. Im not sure its even possible to run a boat at stochiometric ratios since they are under full load at all times. You should definitely run oil with ZDDP in it. I run high mileage blended oils 10w30 with ZDDP additives in my marine 305 chev.

A word on the dipsticks. My boat has 2 dipsticks. One is the oil change dipstick which runs down under the oil pan and connects to the actual drain hole. The 2nd is on the other side just like a car into the oil pan. My oil change dipstick does not read the same as the one in the oil pan. It does not self level with the oil when i compare them. It always reads full. Even when it is low. Why? I dont know. I can only speculate that the dipstick may not let air in and holds a vacuum on top of the oil. Or the surface tension and skin friction in the tiny tube is to much to self level. If you have the option i recommend you get an automotive one and instal it.

Im also curious if the oil manufacture said anything about running fully synthetic oils in a hydraulic flat tappet engine.

nigelbb wrote:

After I had V8 Developments rebuild the engine in my P38 Ray's recommendation was to use Valvoline VR1 20W50 changing oil every 5K miles.

I made a post about my camshaft rattle and what oil to use, and I had briefly mentioned the same oil. My engine was rebuilt, and it was also recommended to use that very one. What concerns me is that my car does not display “oil pressure low”, yet the dipstick shows nothing. It doesn’t help that the car was on a minuscule incline, but the engine was cold for an hour, so I’m not sure what to trust. I’ll have to get the car on flat ground and assess properly. I may also trial another oil - Smith and Allan 20W-50. The only time the "oil pressure low" light illuminated was when I was going downhill and taking a roundabout at a bit of speed. Never has it come on since.

It also doesn’t help that the previous owner didn’t include a temperature sensor when upgrading the header tank, albeit being a better material. Time to use an OBD scanner for my CarPlay system. Oh, I also have a coolant leak, which is great. Potential lack of oil + a coolant leak is an interesting situation lol.

@p73990
Thanks for a reply from another boat owner. My boat has a 1996, 4.3, V6 Mercruiser which is 3/4 of the same engine as yours (or 3/4 of the 350 version of your engine anyway). Being a '96 it is the Vortec version with roller followers. It was bought by me described as cosmetically perfect but with a blown engine (so right up my street). Engine had been run out of the water so both heads were cracked, 1 piston burnt and 3 bores badly scored. Engine has been rebuilt (replacement heads, bored to +30 thou, new camshaft bearings, crank ground, new oil pump, etc) so as everything was new I put cheap 15W-40 semi synthetic in it. Now it has been run for a few hours and is loosening up nicely, I will drain that out and put something else in. Do I need to worry about ZDDP additives or can I carry on with the semi or even a full synthetic? When the same engine is fitted into a 1/2 ton Chevy truck, they recommend 5W-30 but I'd far rather use something a bit thicker in the boat.

My understanding is that ZDDP is needed for flat tappets...

v8vroom wrote:

What concerns me is that my car does not display “oil pressure low”, yet the dipstick shows nothing. It doesn’t help that the car was on a minuscule incline, but the engine was cold for an hour, so I’m not sure what to trust.

There is no display for Oil Pressure Low, only the light, and if nothing is shown on the dipstick, you've got no oil in it!! It should show between the two notches, the top one being Max and the bottom one being Min. Any less than that and I am not surprised you have tappet rattle. The oil is circulated around the engine so once the top end is being lubricated and the hydraulic tappets filled, you've got nothing left! Fill it up to the Max notch. What oil you use is pretty much irrelevant if you don't put enough in it.

The only time the "oil pressure low" light illuminated was when I was going downhill and taking a roundabout at a bit of speed. Never has it come on since.

That's because you have very little oil in it and a combination of downhill and turning meant there was nothing left in the sump for the oil pump to pick up. Keep running it like that and you will very soon be looking at another engine rebuild.

It also doesn’t help that the previous owner didn’t include a temperature sensor when upgrading the header tank, albeit being a better material.

There never was a temperature sensor on the header tank, the temperature sensor is on the top of the inlet manifold. It is two sensors in one housing, one to feed the engine ECU (the temperature you see from OBD) and one to feed the temperature gauge on the dash.

Oh Yea. I forgot they changed to vortec engines in the 90s. So yea ZDDP is not needed

Gilbertd wrote:

v8vroom wrote:

What concerns me is that my car does not display “oil pressure low”, yet the dipstick shows nothing. It doesn’t help that the car was on a minuscule incline, but the engine was cold for an hour, so I’m not sure what to trust.

There is no display for Oil Pressure Low, only the light, and if nothing is shown on the dipstick, you've got no oil in it!! It should show between the two notches, the top one being Max and the bottom one being Min. Any less than that and I am not surprised you have tappet rattle. The oil is circulated around the engine so once the top end is being lubricated and the hydraulic tappets filled, you've got nothing left! Fill it up to the Max notch. What oil you use is pretty much irrelevant if you don't put enough in it.

The only time the "oil pressure low" light illuminated was when I was going downhill and taking a roundabout at a bit of speed. Never has it come on since.

That's because you have very little oil in it and a combination of downhill and turning meant there was nothing left in the sump for the oil pump to pick up. Keep running it like that and you will very soon be looking at another engine rebuild.

It also doesn’t help that the previous owner didn’t include a temperature sensor when upgrading the header tank, albeit being a better material.

There never was a temperature sensor on the header tank, the temperature sensor is on the top of the inlet manifold. It is two sensors in one housing, one to feed the engine ECU (the temperature you see from OBD) and one to feed the temperature gauge on the dash.

Thanks, this makes a lot of sense.

Why I assumed the car had no "temperature sensor" is because it was aftermarket and the needle would start from 0 and stop just before "half", so I assumed it was not operating as standard. That is somewhat reassuring - knowing the car itself was not overheating. I've ordered oil today, and to my surprise, it will be arriving tomorrow. I'll gradually fill it up on flat ground and use the dipstick as a measure of where to stop.

I should note, the tappet sound was there when purchased, and when my mechanic had inspected the car, so not a direct link to low oil.

v8vroom wrote:

Why I assumed the car had no "temperature sensor" is because it was aftermarket and the needle would start from 0 and stop just before "half", so I assumed it was not operating as standard.

It is only supposed to go up to half way up the scale, when it goes beyond half way is when it is starting to overheat so that is when you immediately turn the engine off and start to worry.

It takes a litre of oil to go from Min to Max on the dipstick, so if nothing is showing, I'd start by putting a litre in, wait for 30 seconds or so for it to drain down into the sump and then check it and add more if it needs it.