Why Did Chris Craft Change For 40 Wt TO 30 Wt Oil? It’s Actually Very Simple.


Pre War H – 40 wt

A Simple question on the Boat Buzz the other day about using 30 wt vs 40 wt oil came up. It got me thinking that there might be more to the story why Chris Craft changed the weight need in the mid 1950’s. So I did some investigat’n. And low and behold there is a reason. Mmmm, and it has to do with WAX! It all has to do with organic chemistry. Now many of you may be thinking. Oh noooo Organic Chemistry and Woody Boater… This is going to be a mess. The good news, is no one will know if the big words are spelled right.

40 wt and 30 wt – Pre war- Post war

See crude oil contains many products from volatile gases, which would be at the top of the list, through fuels like diesel, gasoline, kerosene and then lubricating oils and greases, ultimately ending up in the heaviest components like tars used for road surfaces.

Oil pressure gauge

Crude oil is where it all starts and the process of fractional distillation extracts all of these and many more hydrocarbons. Paraffin which also exists in this chain was added to help as a viscosity controlant.  One of two negatives of paraffin is that it did not work well at higher temperatures to maintain a high viscosity, hence oil coolers.  The second negative being that after a while the paraffin would drop out of suspension thus causing sludge in the oil pan and throughout the engine.

Use the right oil.

By the 1940’s there were better viscosity controlants discovered, replacing the paraffin in the oil.  A 30 weight oil of later years would maintain its viscosity better than a 40 weight would have in the 1920’s or 1930’s.

There are many other additives in oils such as dispersement agents, anti- foamants, anti-oxidants in detergents.  If you want to get a leg up on this subject, you can read about aromatic and aliphatic hydrocarbons.

See! it’s simple. Here is the version that readers Digest would do.

Old additives would not hold up as well as new ones. So a 40 wt oil after 10 minutes of running would be like 30 wt. Thats it. So all along 30 wt was the correct type. This is why oil coolers were used. TO keep the wt stable. Thats it!


30 replies
  1. mahoganymadness
    mahoganymadness says:

    That’s a Heavy weight subject to kick of a Monday with..should have went a little lighter, to grease the skids …

  2. Matt
    Matt says:

    My head still hurts. I have good friends, and both grandfathers were engineers. Oddly this all makes perfect sense. The part that was new to me was how the history of crude oil extraction would be a part of our history! Cool stuff.

  3. Capt. Cranky
    Capt. Cranky says:

    One more engineer raising his cup of morning coffee to the “mad-man” for a well done explanation….it also answers a few questions about why the coffee in the office tastes that way it does! LOL

  4. Troy in ANE
    Troy in ANE says:

    Harrison will know if the big words are spelled correctly.

    Very cool explanation, it even makes sense. My (engineer) son keeps asking me why I keep using a single grade oil and not change over to multi-grade (10-30). All I can really say is because the book calls for a single grade oil. I like the Brad Penn products, I guess they are now labeled Penn Grade.

  5. Les Best
    Les Best says:

    Pennsylvania crude is a paraffin base, and Texas crude is an asphalt base, and shouldn’t be mixed. Stick with 1 or the other. Penn brand is the old Kendall, refinery is in Bradford, Pa. New oil is getting crazy, 0w-20, 5w-20, etc. Semi synthetic, full synthetic, high mileage, it makes your head hurt. Great Monday.

      • Matt
        Matt says:

        Oil coolers keep the Wt more stable back in the day, but not needed on our engines since they do not run very hot. This was going to be part of the story as well. As I recall this is also why no need for two wights. 10 W 40 etc.

        • m-fine
          m-fine says:

          No no no! The no need for two weights is because we don’t tend to boat in very cold weather. When it is -20 out and you start your car, you want the oil to still flow while engine warms up. If you start with cold oil the consistency of Vaseline, your engine may not run long enough to get warm. Or there may be too much resistance for your old starter and cold battery to spin fast enough to get started.

          Once the engine is warmed up, especially with a thermostat, the oil is behaving like the second viscosity summer or winter. That is also why there is no harm in using a multi weight oil, especially if you push the shoulder seasons and try to start your boat when it is 32 out.

  6. Mark in Ohio (sometimes da U.P.)
    Mark in Ohio (sometimes da U.P.) says:

    Im finding it harder and harder to find a single grade oil. 10w 40 is what I have to use now. Some people have told me that with modern oil, if I only put 25 30 hours on my engine. I do not need to change my oil every season. I think they are wrong. I do not want old oil laying in my engine for 8 months.

    • m-fine
      m-fine says:

      The oil will become increasingly corrosive with use, so it is best practice to change the oil in the fall before winter storage so the engine is protected by non corrosive oil. With modern oils and only 25-30 hours of use, there is probably little practical need to change every year but it doesn’t hurt other than small punch in the wallet.

      There is no real need to chase down single weight oil. Get the upper weight and zinc correct first. A heavy duty marine/truck diesel formula second. If you run in cold weather, use multiweight, if warm weather only, either multi-weight or single weight will work.

      • Mike Fogarty
        Mike Fogarty says:

        Is that ND oil with zink? I need ND in my Gray 1955 6 which has never been overhaled. I cannot find ND with zink. What brand is it? Thanks, Mike Fogarty

  7. matt
    matt says:

    Rotel by Shell makes a great streight weight oil. Tractor supply and other farm type places sell it for farm equipmtent. Also found it my local hardware store. I buy 12 1 gallon jugs so I have it always

  8. Dino - KX6D
    Dino - KX6D says:

    I may be preaching to the choir so indulge me…

    Marine motors see a constant load and require a “heavy duty” oil. Diesel oils meet this need, even in a multi-viscosity. Shell’s Rotella was already mentioned. I use Chevron Delo 400 15W-40 in my diesel truck, my gas Jeep, and all of my boats. (The wife’s Honda gets 0W-20 Mobil 1 per the mfg).

    If my boats lived in the water I would run a marine oil which uses an additional additive to control water. Since they are on trailers and serviced regularly, the diesel oils (compression ignition), which are also rated for “spark ignition”, my engines are very happy.

  9. Brian Robinson
    Brian Robinson says:

    Ted Cartner, who builds some of our engines said to use Valvoline motorcycle oil designed for wet clutches. It comes in 10w-40 and 20w-50 and has 1000ppm of zinc. It’s cheap and available everywhere. So that’s we use in the Hercules engines. We used to use Rotella or Delo but they lowered the zinc so need zddp additive now.

    • Ronald
      Ronald says:

      Brian, I have read that about Rotella in British car forums a few years ago stating after some certain date that Shell reduced the amount of ZDDP and it was no longer as good as it was for older cars and boats etc. I have been using Castrol with additive for a few years now and changing it in the spring before the season with no problems. I understand there are many differing opinions about when to change oil, weight and brand but as I read in a motorcycle magazine tech article once that the main thing is that you are changing it and not so much the brand.

  10. Dan T
    Dan T says:

    Useing the correct oil is important, but I think down the road, finding ethanol free gasoline will be more of a problem. The allowed amount of ethanol has been increased from 10% to 15%. I know in some places non-ethanol is still available and hopefully it’s not getting phased out completely. Ethanol causes a lot of problems and can destroy older marine engines.

    • Brian Robinson
      Brian Robinson says:

      We haven’t been able to buy non-ethanol anywhere in California for three years. It Sucks. Literally every boat that comes in the shop has fuel issues now.

  11. Phil W
    Phil W says:

    Summit Racing has a 30W Hot Rod oil with plenty of zddp for our uses. Seems to work well in the 48′ Gray.

  12. Johnny V.
    Johnny V. says:

    In our area of Upstate NY non-ethanol gas has been getting easier to get. Like most, I use it in everything except for the road vehicles.

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