2018 WRX Pennzoil ultra platinum 5W30 - 3050 miles

[twX]

I'm not sure that early oil changes have any benefit when it comes to fuel dilution. According to this study on fuel dilution in direct injection engines (2002), fuel dilution almost reaches a steady state level within a few hours of engine operation. From there it will rise when conditions for dilution are favorable (cold engine, high engine load), and then drop when conditions for dilution are unfavorable (warm oil/coolant, low to moderate engine load) as the fuel vaporizes from the oil.

The figure below shows fuel dilution in an engine operating with 40 degree C coolant/oil temperature for 10 hours, followed by 10 hours of 80 degree C coolant/oil temperature, with moderate engine load at 2000 rpm:


Within two hours of "highway cruising" conditions, dilution drops from over 7% to under 3%, and drops further to 1% after 10 hours. I'd expect that some of the heavier components of the fuel will never evaporate. This is not obvious in the 20 hour test, but might be in a 100+ hour test.

The best way to combat fuel dilution is to avoid short trips with a cold engine. High engine torque increases fuel dilution a lot as well since the injectors are firing for a long time while the piston is travelling far down into the bore, washing the cylinder walls with fuel.

Preheating the engine with a block heater in the winter, and using a grill block to reduce warmup time and increase steady state oil temperatures should help as well.

A functioning PCV system should better allow fuel vapour to escape from the crankcase.

 

[OVERKILL]

I'm not sure that early oil changes have any benefit when it comes to fuel dilution. According to this study on fuel dilution in direct injection engines (2002), fuel dilution almost reaches a steady state level within a few hours of engine operation. From there it will rise when conditions for dilution are favorable (cold engine, high engine load), and then drop when conditions for dilution are unfavorable (warm oil/coolant, low to moderate engine load) as the fuel vaporizes from the oil.

The figure below shows fuel dilution in an engine operating with 40 degree C coolant/oil temperature for 10 hours, followed by 10 hours of 80 degree C coolant/oil temperature, with moderate engine load at 2000 rpm:


Within two hours of "highway cruising" conditions, dilution drops from over 7% to under 3%, and drops further to 1% after 10 hours. I'd expect that some of the heavier components of the fuel will never evaporate. This is not obvious in the 20 hour test, but might be in a 100+ hour test.

The best way to combat fuel dilution is to avoid short trips with a cold engine. High engine torque increases fuel dilution a lot as well since the injectors are firing for a long time while the piston is travelling far down into the bore, washing the cylinder walls with fuel.

Preheating the engine with a block heater in the winter, and using a grill block to reduce warmup time and increase steady state oil temperatures should help as well.

A functioning PCV system should better allow fuel vapour to escape from the crankcase.

Unfortunately, given the age of the study and the fact they used a "prototype" GDI engine that doesn't appear to fuel dilute more than its MPI counterpart, I think the takeaways we have from it are limited.

As we've seen, some engines (Honda) fuel dilute a massive amount while others (BMW), don't. Others are somewhere in the middle.

 

[bluesubie]

What does Subaru recommend in terms of OCI? 3K is just so short if you are a highway commuter but clearly you have some basis for keeping the short OCI.

Subaru of America went with an OCI of 6,000 miles for normal conditions and 3k miles for severe conditions (regardless of engine type) back around MY2011. Prior to that turbo's required/recommended the severe condition OCI but not since then.

 

[Nexoidian]

I am thinking I will run this OCI 4-5000 miles. I have a quart of HPL engine cleaner in it this time. So I plan on changing the filter at 2,000 miles. Then going at least 2,000 more. May go to 5,000. I will send a sample from this run in when I do change it.

Next oil change I was thinking of going with Pennzoil Platinum Euro 5w30.