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Two Stroke oil in your Diesel and outboard fuel.


I went to purchase some Two Stroke Oil as we are about to fill up with diesel fuel. I regularly add TSO (Two stroke oil) to our diesel fuel at a ratio of roughly 200 to 1. It certainly makes a noticeable difference. You may ask why not just add engine oil and I understand that the answer is that two stroke oil will not settle out the same as engine oil. (Any oil experts out there with more info please email me)

I have tried to run without several times and quickly notice that the engine becomes noisier and runs erratically.

We are convinced that on our 1990 engine it works well. It replaces any lubricity lost by having to use low sulphur fuel. (See the page on Diesel Fuel)

When I went to the outboard motor shop I found it is run by a technophobe who immediately asked what type of TSO I wanted. What do you mean? I asked.

I then had a really interesting discussion about the virtues of various TSO.

There are two basic types. First, there is oil for air cooled, high revving, hot running engines such as lawn mowers, strimmers and chain saws. This apparently has aluminium added for lubrication. When this is used in a marine two stroke engine it can cause problems like plug fouling.

The second type is TC-W3. This stands for Two Cycle Water Cooled. This apparently contains ash for lubrication. The 3 stands for the grade. There is a grade 2 which is thicker. The grade 2 must not be used in an engine designed for 3 but the 3 can be used in an engine designed for the thicker grade 2. This TC-W3 is for use in engines which run considerably cooler than lawn mowers, strimmers and chain saws.

Therefore the local shop only sells the grade 3. ( TC-W3 ) They also recommend never using anything other than TC-W in an outboard and recommend the TC-W3 as this is universal.

Your outboard manufacturer will have a recommendation of which oil to use. Stick to it - even though in some supermarkets and stores much cheaper versions can be found. Most of these are the wrong type to use in a cool running marine engine. If you can't find which grade to use then TC-W3 seems to be the safest.

So the question then was which to use as an additive to Diesel oil. After a couple of hours research on the web, it seems that most people are using TC-W3. This seems to burn the cleanest, so will not carbon the engine or produce harmful exhaust emissions. Actually it is claimed to help keep the engine clean and reduce exhaust soot.

The one other factor was whether to use mineral, semi synthetic or full synthetic. It seems that full synthetic does not work so well. Therefore we are going to continue using either mineral or semi synthetic TC-W3 as an additive to our diesel fuel at a ratio of 200 to 1.


The final bit of advice that I found is that if your diesel engine is manufactured after 2006 or has single rail injection then additive should not be used. It seems that since then the engine manufacturers have started using materials in the system to allow for the lost lubrication of low sulphur diesel fuel.

I have checked what others are doing and the general ratios most people are using as an additive in diesel fuel are:-

1 to 200 or amounts roughly as follows:-

1fl oz per gallon

1fl oz = 28cc

5cc per litre

25cc per gallon

This does make filling up with diesel a bit expensive but should help with the longevity of your engine.