Oil/Fuel Dilution Concerns...

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[32spoke]

There's lots to unpack here. I'll chime in on what I can help with.

It's expected that your oil will be thin, have fuel contamination, and metallic content during the first say 1600km (1000m). I'm not surprised at all. This is when most of the wear your engine will ever see occurs. During the break-in period.

Fuel dilution will increase as the temperature decreases. This is due to a number of factors, which I may go into more detail on in a future thread. They are: slower vaporization rate of gasoline as it enters the cylinder, thermal deformation of pistons relative to cylinders, and the time it takes to engine oil to reach operating temperature (80-100C). Some people may have no issues at all with fuel dilution if they live in a warm climate.

Viscosity shearing is not what's negatively affecting your oil. Shearing is where the viscosity modifiers (very specialized chemistry that is designed to increase the viscosity of the oil as it gets hotter) are physically SHEARED and no longer can perform their function. These molecules actual unwind and take up more space as they get hot. They prevent to oil from thinning out too much as it warms up. Timing chains are a big source of shear wear. If you're concerned about shear wear look at the HTHS (high temp/high shear) testing values for given oils.

One must be careful using a grade of oil other than what is specified for the engine. if you want to try it (and I will be in the future), I suggest focusing on the winter grade value (the "W" part of the viscosity range. Fuel dilution is really only an issue when the engine is below operating temperature. This has very little to do with coolant temperature an everything to do with oil temperature. Just because your coolant is at 80C, doesn't mean your oil is. In fact, in my test, oil temp takes quite a long time to reach 80C and above, and requires a fair bit of time (say 30 minutes) or considerable load on the engine. I may get into why 80C is the magic number in a future thread if anyone is keen. But the W number is important. Toyota likely runs 0W oil fortwo reasons: 1) to ensure the engine starts easily in VERY cold weather (think -45C) and 2) to improve corporate average fuel efficiency numbers. Going down the grade scale by one grade can increase fuel efficiency by between 0.2% and 0.4% which seems like a small amount, but in an ICE engine that is likely only 40% efficient, it's a huge deal.

Back to why focus on the W aspect of the multiviscosity oil...

Firstly, fuel dilution is MUCH worse at low temps so that's the end of the scale we want to modify. Secondly, if you go up a grade you're exposing the oil to a viscosity range that it already sees within the manufacturer recommended oil. Meaning, 0W-20 "contains a range of grades, between 0 and 20. So introducing say a 5 weight oil isn't an issue. It won't offer more protection to fuel dilution. That comes from chemicals in the oil called dispersants. Any oil with API-SP rating will have a butt load of dispersants. It may REDUCE fuel dilution because it will form a stronger film between cylinder bore and piston rings when the engine is cold as compared to 0W. When an engine is cold, the parts don't fit together properly. They all have clearance values determined by the engineers, and those clearances are wider when the engine is cold. All metals expand (albeit at different rates) when they warm and the engine is designed to run at operating temp (80C+). A 5W oil will provide strong er film strength at startup and may reduce fuel dilution as a consequence. 5W is rated for-40C (this spec published in SAE J300 if you want to look it up) and 0W is rated to -45C. So unless you live somewhere REALLY cold, 5W-20 should be fine. Even the owners manual says you can run it (but you should change it back to 0W-20 when you can).

If you go up to XW-30, you're introducing an oil viscosity range that the engine wasn't designed for. Remember those clearances I talked about? They could be too tight to allow adequate film formation with your 30 weight oil in some journals or bearing surfaces. I suggest you steer clear of increasing that end of the range.

Sorry for rambling on, but I hope this helps you work out your plan of action.

Toyota recommends 5w30 for the T24A-FTS in non North American applications.. which undermines your comment about xw-30 oil viscosity. 0w20 oil is used to bump up mileage for improved CAFE ratings, which is why Toyota recommends this oil- yet Toyota even recommends a 10w30 oil as well for this engine, though obviously not suggested for cold weather.
https://toyota-club.net/files/techdata/ttx/lc_250g.htm#:~:text=Toyota-Club.Net - Fluids,Brakes

https://bobistheoilguy.com/forums/threads/t24-a-fts-turbo-engine.400187/