Go to the top of this page. it is the fifth one ,open it

I don't know about modern engines but on old ones such as Fox McCoy K&B, even 5% synthetic keeps the dread castor crud at bay. I use a synthetic sold by Halford's in the UK. I was told by a knowledgeable fellow that it would work very well on my old two stroke outboards. He regularly flies jets and he uses this stuff at a fraction of the cost of the expensive synthetics. If it works in jets then it should work well with anything.

Andrew.

     Hi Eddy R. et. al.:

     I have read the above article on engines by Randy Smith and it suggests that I use 25%-50% synthetic oil in my fuels.

     I guess what I am asking is what is the least amount of synthetic oil do I need in my fuels to keep the engines clean.  Is it the above stated percentages or is there a lower amount of synthetic oil that will do the job?

    Has anybody ever done a test to determine the answer to this question.  If so what were the test conditions and the results?

    Perhaps a synthetic oil that is sold for outbard engines, chain saws, lawn mowers etc would  a well as Klotz.  I  have no idea if these oils would be sufficient to protect our model engines.  Once again, has there been any  testing of these oils in our model engines?  Amsoil talks a good game!

    George Aldrich's "Magnum" fuel reportedly used the "50:1" synthetic at a very low content (maybe 5-8%, but I never did a boil-down test on the gallon I had).

    For most current ABC/AAC engines I think even 50% castor is way too much. How much you use seems to result in a slower or faster rate of varnishing, but it eventually varnishes up. For conventional stunt engines like the PA/RO-Jett I think you could run NO castor at all and likely be better off, as long as you used after-run oil religiously. Like SIG Syn-Power. It also reduces the oil drag which has substantial effect on the run quality - the less oil drag, the more it will respond to load, the more, the less it will respond. You can use that to your advantage - as you can read about in the next SN "IC Engine" column. Or in this thread:

http://stunthanger.com/smf/index.php/topic,28567.0.html

    The viscosity of the fuel (mostly due to the total oil content and the type) can also make a huge difference in the way the engine runs. For the most part, the thinner the better, but you have to consider the oil drag effect, too. It appears from extensive testing that the drag in the fuel supply lines can dramatic effects on the way it runs, particularly on the larger engines. David and I learned that lesson from the 4-stroke guys, and from comparing various fuel system parts from different engines.

  I wouldn't use straight synthetic in any of the current "economy" engines like the LA/FP, they don't seem to suffer too badly from varnishing, and the conrod has to stay alive somehow. But you don't need 25% total content either.

   For older engines like Fox/McCoy/Johnson etc, my advice is to just use castor and live with the side-effects. The McCoy and Johnson actually require carbon buildup and lengthy break-in to fill the pores in the sintered pistons. And as Randy's fuel article pinned to the top of the forum notes, running synthetic after you run castor can remove the carbon that forms the piston/liner seal.

     Brett