FYI Murrays Autodiscount sells Amsoil 10w30 and 10w40 in my area for $ 5.99/qt.
Since as we know that "film strength" is most important in synthetics the latter number is of most concern. I have some boating friends who use Amsoil 10w40 and it works fine in their big blocks. Concerned about using lower weights in synthetics vs. heavier weights in conventionals? Think about this. The 2004 Dodge Viper V-10 500HP comes with a "factory fill" of Mobil-1 10w30. This viscosity in synthetics is popular with Benz and Porsche as well. This is stuff that has been tested and is very compatible with engine "seals". I think the myths about synthetics are diminishing and the truth is finally upon us. I never mind spending extra if I know what I am getting is a good value. I use marine engine Amsoil for the extra corrosion additives they add in there for the marine environment/ condensation issues being greater. I will say that I have used Mobil-1 15w50 and later 10w30 and never had a problem. But i read where somebody pulled an engine apart that had used it and there was sludge. So I got fussy and switched to Amsoil marine due to low volitility at high ( 200 degrees C) temps because the lower the volitility the less the chance for sludge formation due to the oil "boiling off" and concentrating into sludge. The cool thing is price wise most synthetics are within a dollar or so of each other. Now the premium filters run around $ 11 bucks each but they remove the stuff that causes lots of wear. I would say I spend about $ 42/engine using Amsoil and a Mobil-1 filter and do it myself per manufacturers drain which if I recall is every 20 hours. So for twins changed twice a season it totals $ 168.00. WIth conventionals it would be $ 60.00.
SO it costs me $100.00/ season more to do it my way.

Zachsam,
Why don't you recommend anyone using 20w50??
My RP rep highly recommend running Racing 51 which is designed for drag racing, marine and truck pulling. It has a viscosity typical of a 20w50 oil.

My opinion is that a user doesn't require such a heavy oil to provide protection, especially with an oil that provides high film strength. RP didn't offer a racing 20w50 when I had my distributorship (not that I would have sold it). I kept very, very little 20w50 in stock. In all the training that I had received during my trips to the factory in Humble Texas, and with working with the tech guys (David Canitz , Mike Crosby, Jody Williams) as well as quite a few of the other more knowledgeable distributors, we always advocated lower viscosity for higher hp and better cooling, without sacrificing protection. Although 20w50 was recommended for my little brothers flat 6 (911), which relies on an extreme amount of oil to handle both cooling and lubrication, he has been running RP41 (15w40) for many years now without incident (lots of trips to LOTO from St. Louis). But that's just one data point, which doesn't really mean diddly.

Maybe I've been away too long and I need to be re-edumacated.....

My understanding is that road racer Paul Gentillozi pushed them to develop RP51, but I honestly haven't kept up with it the tech side of it.

If you trust your rep, use his recommendation....one question - is 20w50 one of the viscosity's recommended by your engine manufacturer????

Thx,

Steve

Active Mike:

Let me back step slightly.....my comment about 20w50 was made in the context of your aspirated motors, and it's my opinion only.....there are indeed instances where a heavier oil was offered by RP (for instance, Top Fuel cars....I can't recall the weight, but I think it was something like 70). My original comment was done in haste.

Thx,

Steve

What do you guys feel about using a good diesel oil... 15w40... They have the same good additives that these big blocks need... Good anti-foam, great with heat....

Any one dare to answer?

Now your making my head hurt trying to think back to my oil "daze". I'm sure just 'bout every manufacturer, including RP, will tell you that oils are formulated for specific applications, with certain additive compositions that enhance it's ability to perform in that application. Would I personally be afraid to use a diesel oil in a non-diesel app....no, but that's only my opinion, and it's really not providing a technical response to your tech oriented question, so I'm admitting that I'm pretty much wussing out and just killing some time here.

Steve

Well, on the bottle is says its is also good for gas engines... LOL.... I have no clue...

What is NMU?

I mixed a lot of strange concoctions up on my personal vehicles just to see what happened....I probably had the only Legend and Z running around with a modified version of RP9 (0w10) in both the engine and tranny. Never caused any problems, but it clattered like a diesel and affected my vacuum.

Steve

Northern michigan university....... Its a long story on the name... While i was there we had 2 years of record snow fall...You had to recognize houses by thier rooflines....


I think Im going to stick with mercs 25w40......

According to Larry Watson, consumer affairs manager for Mercury Marine, 10 years ago MerCruiser recommended single-grade 30W or 40W SF-rated oil. Today, the recommended oil for use in all MerCruisers is Quicksilver 25W40 CD or CF certified multigrade oil (Quicksilver part number 92-832112A12). The use of other brands with the same certified viscosity and service ratings are probably satisfactory. Synthetic oils are excellent products and well-accepted in automotive use with no adverse effects, however, they have not been "qualified" by MerCruiser for use at this time. Wonder why?