I'm now putting 20w50 in my Prius. Arrrrh!

Actually, most oil pumps are essentially a constant flow device, up until the bypass pressure. After the bypass pressure is reached then of course flow is reduced, (as some of the flow is bypassed).

The difference between a 30 weight and a 40 weight oil is nowhere near enough to cause it to go into bypass at normal operating temperature, though you are correct that it will cause it to go into bypass more readily at cold start temperatures. You may however be underestimating the effect of relative small temperature differences on reversing this situation.

Believe it or not, a temperature difference as little as 4 or 5 C is enough to account for a one full grade difference in the oil at typical cold start temperatures. So for example a 5w30 oil at 0C would be a similar viscosity to a 5w40 at around 4 or 5 degrees C or a 10w40 at about 8 or 9 degrees C. If you think that I might be getting inadequate oil flow with 5w40 and ambient temperatures always above 10C, then please never drive your car at temperatures lower than 5C (with the recommended 5w30), as you WILL be in a worse situation.

BTW, you can verify the above by taking a look at the widman viscosity calculator here: Graph your oils | Widman International
Put in the following values to compare with the typical 5w30 and 10w30 oil data supplied there:
5w40 : 79/13 (@ 40C/100C)
10w40: 96/14.3 (@ 40C/100C)
These are only typical values of course (these two are actually for Shell helix 5W40 and 10W40), as oil grades cover a range of values within each grade. Also, the widman calculator is only extrapolating based on typical oil viscosity index parameters, so the data should be interpreted as "typical" rather than an exact representation of any given brand oil.

The bottom line is that oil grade requirement cannot be totally separated from ambient operating temperatures. I know that in the US most manufactures like to have a uniform oil grade specification (like the "one and only" correct grade), but in many other places manufactures often recommend a range of acceptable grades, based on expected ambient temperatures. For example the following oil chart was included in owners manual of my 2005 Prius (actual scan of my manual). This is the official Toyota (Australia) oil recommendation for my Prius.

 

Yes obviously it's a function of speed, but I'm pretty sure that everyone else here understood that we were talking about flow vs viscosity, not flow vs speed. The flow is also a function of the size of the oil pump, but that's not relevant here either.

 

Actually high mileage oils seem a bit scarce here. I did look quite a bit, but in the end the only thing I could find locally was the Valvoline Maxlife 20w50. That's why I blended it with the thinner fuel conserving oiI that was causing my increased consumption. Anyway it worked ok, I only blended in about a liter and it reduced to oil consumption without making it too thick.

Anyway, that oil is drained out now (as of about December) and I'm back to running the usual 5W30 in the more robust A3/B4 rating rather than the GF5 fuel economy rating.

 

I'm just updating this thread with a new development in the oil consumption situation. I was going to wait until I had a bit more data, but basically it seems that after having the hybrid battery replaced (with a new one from Toyota) that my oil consumption has dropped very dramatically.

I'm putting this here because I had posted elsewhere about my recent sudden decline in oil consumption, and I don't want people to jump to the conclusion the just because I blended a little bit of high mileage oil (which is supposed to condition seals) on my previous oil change interval that it's caused this reduction. I accept that it might have played some small part, but I really think it's more related to the new hybrid battery. It's really quite noticeable that the engine doesn't need to work as hard now (with the new battery) and in particular it doesn't need to spin the engine to dump charge when I descend big hills.

What happened was that late Nov last year we had the main battery replaced. A week or two later I changed the oil because it was due (no additives, just the same 5w30 that I usually use) and weirdly it has hardly used any oil since that change. Unfortunately it's hard to pinpoint exactly why this has happened because over that time three things have been different to normal.

1. The new HV battery.
2. The fact that I'd had some of that high mileage oil in there over the previous 10,000 km.
3. My wife was on holidays and the Prius (her Prius) wasn't driven on the same terrain etc as normal (though still driven nearly 4000 km).

Number three is back to normal now, so I should be able to rule that out (or in) as the cause over the next few weeks, but there's not enough kms yet for me to say whether this had any effect.

Anyway, it was so unexpected to have the oil consumption suddenly reduce to almost nothing on this Prius after 13 years and nearly 320,000 km. Pleased that it happened, but still not totally sure why.