Originally Posted by Dogface
Originally Posted by Strut10
I know some of the older 'Rudes were 24:1.
I'd hate to try running one on 50:1 (synthetic or otherwise) and be wrong.
I'd try like heck to find the manufacturer's spec mix ratio and run that.
Just an FYI. I bought a new Johnson in the early 80's. It was a 100/1 mixture. I had it about a year or two when the dealer called and told me that they changed the specs to 50/1. It is still running.
This site will give you a little history and help on the Johnsons and Evinrudes. The fellow who wrote explans some of the change to oil- gas mixture. http://www.sschapterpsa.com
Then from 1985 to 1988, OMC actually recommended a 100:1 mix, this was written on a decal placed on motors of these years. Observed on one 1985 9.9hp & the 1988 Evinrude 15hp shown in the photo below, there was a sticker on the powerhead cover next to the fuel line connection which shows cartoon icons of a fuel pump, and oil drop, showing a ratio of 100:1 for the fuel to oil ratio.
1988 15hp Evinrude showing 100:1 fuel ratio symbol
Apparently there was some problems with some motor users (probably higher HP than what we are referring to here) & OMC issued a Technical Bulletin #2162 dated March 1986, for the mechanics to remove the 100:1 decal, replace it with a 50:1 new decal & inform the owner, when the motor was serviced in an OMC repair shop. This was recommended for rental, commercial & heavy duty service engines.
The factory apparently did replace some damaged power-heads because of this. Apparently some motors, under the right (or wrong) conditions , if ran at a high RPM or under a load for extended periods of time, may seize due to lack of adequate internal lubrication. There was another bulletin sent out in September of 1988 that kind of soft pedals around the issue, for a link to this bulletin #2211 CLICK HERE.
Back when OMC re-introduced their 50:1 ratio of oil, an old time marine mechanic was told by factory engineers that their motors would survive on 100:1 but they couldn't trust people to measure it accurately or control what kind or the amount of oil they used. So they reverted back to 50:1 ratio. The 100:1 was sufficient lubrication for most motors while running. The bulletin had nothing to do with lubrication while running and everything to do with storage of the motor. So the 50:1 you have a margin of safety in case of a lean carburetor on a multi carburetor motor or an overheat and it can still protect the clean internal parts while not used for a few months in a climate that can create internal sweating. It appears that the manufacturer's lawyers were very careful to not create the situation where a possible class action lawsuit could not be presented, so they emphasized the storage internal lubrication theory where the average boater could not dispute it.