Mixing of MIL-PRF-7808 with MIL-PRF-23699

[FieldTeam]

I am not sure if this question is appropriate for this forum. I tried Engine & Turbine Engineering forum first, but think maybe there are experts in this forum who have experience with these two oils.

My question concerns the mixing of MIL-PRF-23699 and MIL-PRF-7808 in an aircraft turbine engine (JT3D-7).

On one side I have an engineer who states that we can unconditionally mix these oils in any quantity/ratio, with no documentation or limiting conditions.

Printed data I have found states that these oils can be mixed in emergency situations, but under the condition not to exceed more than 50% of the oil tank capacity and at the first opportunity the oil tank shall be drained and reserviced with the correct oil (42B2-1-1 dated July 2005).

P&W has a service bulletin advising I can change from 7808 to 23699, but have to drain and re-service with 23699 and stencil the tank with the new oil number and document this change. Nowhere does the SB state I can service without condition between 7808 and 23699.

Obviously, either oil can be used. What (if any) adverse effects, both long and short term, could there be with mixing of the two?

Does anyone have any thoughts or input?

 

[romke]

both specifications cover various oils based on synthetic esters with viscosities in the range of 3 to 5 CSt/100 degrees C. they might be readily mixable, but in the long term the behaviour of the mixture might act otherwise in respect to wear protection, anticorrosion capability and combality with seals, plastics etc. given the type of equipment they are used in (aircraft type gasturbines) mixing the two fluids is restricted for safety reasons and changeover from one fluid to another usually has to be well documented. that does not mean they are incompatible from a technical point of view, but in this application obviously safety gets the priority.