what is the recommended engine oil for mercruiser 496 mag ho

25-40 weight Mercury semi-synthetic or full.

How many hours are on it?

The Amsoil Dominator 15-50 runs nice in it. There is zero lifter noise when it starts.

I use Mobil 1 V Twin, very good oil and can be bought anywhere.

I took the advice of a guy on another fourm and started running Valvoline VR-1 Conventional 20W-50 in my 496 H.O. a couple years ago. Motor seems to like it I have no complaints. Only problem with the conventional oil is after about 15 hrs it thins out and you start to lose a little oil pressure. I can actually tell on my pressure gauge when it is ready to be changed which I do not mind. I would rather spend a couple more bucks and change my oil once or twice a year for piece of mind. The conventional oil has one of the highest zinc contents in which is good for your motor, especially if your running it hard. Fortunately for me my motor has the drain hose attached to the oil pan. So at the end of the day I pull the boat out of the water pull my drain hose and go have a beer with my dinner. Put fresh oil in her in the morning and back out on the water.

What about Mobil one 0-40? Anybody have experience? Seem like it would be a good choice for oil cooled blower motor.

The extreme pressures these motors run under I would be leery about a 0-40. Think about it, your car motor turns 1500-2000 rpm at highway speeds, we are turning these motors at 5000 +. This is similar to how it was explained to me. Like I said I would rather spend a little more money on oil changes a couple times a year and run an oil that can stand up to the pressure that I am going to put it through. 10 quarts of Valvoline VR-1 20-50 cost roughly $60 + a filter. That is cheap insurance that I am willing to spend a couple times a year to make sure I am putting something in my motor that will help it last longer......JMO. Read around, oils with high zinc content is good for these motors. Don't get me wrong I am no expert, but I have done allot of reading and have received some good information.

I have good results with VR -1 40 w and Driven motor oil 20w - 50. Driven oil is using the newer ester technology MPAO.

High ZDDP is good to a point, but only in a balanced additive package(it's strongest attribute is protecting flat tappets, but it does form an antwear barrier in other places as well) and too much can be harmful. Really don't see the need for a 50wt oil unless of course it shears a full grade fairly quickly with use. The big block Chevrolet shouldn't have major issues with shear. This engine design is actually very easy on oil from that standpoint. The main thing I would worry about in a marine application would be fuel dilution. I don't like conventional oils due to the large amounts of viscosity index improvers, which is the reason some here are experiencing the drop in pressure.

Mobil 1 is truly a great oil, however I find it makes engines produce more noise, and it has always increased oil usage between drains, and tends to make seals weep oil on countless applications(your mileage may vary and high mileage formulas do not do this). Granted none of these affect wear, but all are aggravating. I prefer group IV/V oils, especially polyester/diester offerings. I realize quite a few here like the Mobil 1 V Twin and I am curious as to why this is? Is this because of higher ZDDP? availability? Or teardown results? To each their own, but I prefer to use something based on data of some sort. Again Mobil 1 is one of the most technologically advanced oils you can buy OTC. Other oils can be ordered that IMO and from teardown are better at certain things. All oils compromise something, just the nature of motor oil. No such thing as "the best oil".

Their 10w-40 non MPAO (whatever that is) is $16.99!!!??? The crazy thing is that it isn't anything brand new. It's used in quite a few OTC oils and JGR probably gets it from Exxon/Mobil. They claim 200 VI, but it's s very heavy base oil compared to a PAO/Ester type. JGR offers (not produces) a stout oil, however it does not stack up to look superior to ester offerings. Everyone likes to use NASCAR when they discuss JGR oil , but what they don't understand is that this is silly. Mobil and other oil companies custom blend oils that would blow your mind for these teams. And several race teams sport a brand but another is under the hood. Prob not telling you anything you already didn't know. I will bet money race teams do not use MPAO base oils.

I'm not bashing JGR oil, just MPAO isn't anything spectacular. Oils are so good these days, it's hard to see them advancing many times beyond their current evolution. Compare early 80's oil tech to early 90's. Talk about an oil renaissance. It's the reason why so many nowadays think all conventional oil is junk and wears more than synthetic blah blah... But some phenomenal formulas were devised back then, especially with synthetics, widening the gap in performace between conventional and syn. That gap is currently the most narrow it has ever been with respect to most applications, besides racing and extremely cold climates, where synthetics shine. The fact is modern conventional oils are very good. For applications like a boat however, especially a HP one, synthetics seem to be the best bet.