This engine seems so new and strong still, I don't know if engine flushes will help or cause more damage. Adding oil doesn't bother me, I've been doing it on my 2006 forever.
engine flush done correctly (just idling a few minutes and engine not grossly sludged) doesn't damage anything. and you have a chance to test a hypothesis and potentially reverse a spiraling chain of events. but, it's your own choice.
Seeing that I don't know the cause, it would be hard for me to try to fix it. I'm trying the natural cleaning method by using pennzoil ultra platinum synthetic oil, it's the cleanest out there. I tried treatments of MMO on my 2006, didn't work at all.
i agree, in my own experience, MMO is near useless. besides, if you wait too long, nothing will fix it, as oil deposits turn into hard carbon with time.
Depending on where you bought it, exchange it at the store. Walmart exchanged mine without a receipt, no problem. That said, I'd guess the mpg hit would be less than 1 mpg, if that.
I'm changing the oil in the Prius today and found some 5w-20 Amsoil laying around in the garage. I'll never use it unless I go with it today. I'm still wondering how much of a MPG hit I'll have with cold starts compared to the 0w-20? It's about $25 in oil, and I doubt I'll burn more than that in extra fuel due to the thicker oil.
I'd give it to you if we were close by. Don't know how I ended up buying it, but my guess is it was incorrectly stocked with the 10w-40 I bought for my motorcycle.
It's been on year since my original post. I've been using 5w20 for two oil changes in the spring-fall. I switched back to 0w20 synthetic in the winter. I don't find any noticeable difference. The higher summer temperatures make the "5w" part pretty unnoticeable. I can't comment on cold starts as I've never used the 5w20 in the winter.
I'll be experimenting with 5w-20 in the winter since I go a year between oil changes. If I notice any differences I'll report it here... in a year.
In case you're wondering, the European manual has much more lenient oil requirements: They do push for 0W-20 though. Semi-synthetic oil is pretty much the default here for any car made in the past 25 years or so. Mineral oil (if that's what you mean by "conventional" or "dino") is for classic cars