Long intervals between oil changes leads to the formation of sludge in any engine. The 1MZ-FE is particularly susceptible to this because of the orientation of the cylinder heads, and the narrow drain passages.

 

So even with synthetic oil, this is still a concern on this engine with mileage?

 

How does your oil look after 10k?
What does it look like inside the oil cap?
What type of driving did you do most of the time during that 10k?
What is the condition and mileage of your vehicle?

If the oil is dark brown/black color, not good!
If it looks like sludge inside, not good!
Mostly in town, short distance, engine not fully warm most of the time, not good!
Vehicle in poor condition and high miles, not good!
Synthetic oil, that's good, usually can run longer with less chance of sludge buildup.

We use our cars very differently, it isn't as simple as amount of miles since your last oil change that can determine how it will affect the engine.
It would be more useful for you to check the oil in your vehicle than wondering what may or may not happen to yours.
Just saying!

 

I can say for sure that 4,000 of those miles are strictly highway, as I road-tripped up the east coast. As for the other 6,000 I can say it’s definitely 50/50. So I’d say 7,000 of those are highway and the 3,000 is city.

 

So most of the driving is highway! Check the oil, see what color it is! If you like, post picture of the oil up here, let others chime in with their comments!
Level as well, after engine is warmed up and allow drain back after engine is off. If the level is still good, that would be an indication that the engine isn't using oil.
On the other hand, if it is low, you may want to pay closer attention to the level from now on, see if the usage is within the acceptable level! Toyota usually consider 1qt/1000mile normal!

 

If this happened just once I think you'd be fine. The 1MZ is a known sludge monster and Toyota down-spec'ed the oil interval to 5000 miles. Given you were running a fully synthetic, just keep up the oil changes in the future. And given the small cost difference between dino and fully syn, just stick with fully syn.

If you're concerned, order a Blackstone oil analysis when you change this 10K old oil. You can collect the sample into a clean container for now while waiting for the Blackstone free container, and get rid of the 10K oil in the engine with a fresh oil change. See details on:

Oil Analysis | Blackstone Laboratories

Learn what's going on in your engine with oil analysis. Our free kits are easy to use and we explain the results in plain English! Since 1985.

www.blackstone-labs.com

 

Don't do it, change it at least every 5k miles or 6months. You will create sludge and start to get sticking oil control rings. These things become a nightmare when you start to get sludge. You don't get the nice, gloppy sludge, you get that hard charcoal stuff that's difficult to erradicate. It gets into every little nook and cranny. My oil pickup tube was jam packed with that hard black crap. I haven't removed the valve covers yet but I wish the previous owner had changed the oil more often.

 

As long as the oil level did not get low, one time at 10K with full syn is no big deal.