The 5-cylinder engines aren't known for their sludge. Perhaps you've only come across models that were poorly maintained. I can't speak for everyone here but I think the concensus would be that the 5-cyl would be recommended over the other engines because it is the simplest and most proven engine option.

Based on Yamaha's good reputation as an engine builder (see Taurus SHO) I would wouldn't rule it out. That engine is so new that we haven't heard much about it here. I think the most talk around here on the XC90 has been about rear wheel bearings. Seems they only have a lifespan of about 50K miles.

The six is an inline, not a Vee. Are you sure the transmission (and it's problems) are specific to that engine? I do not know, but it wouldn't surprise me to hear that the 5 and 6cyl cars use the same tranny.

-Will
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