Newly built engine - no fuel!

Hi,

I recently re-built my 2JZ-GTE engine and annoyingly I didn't realise I was running on 5 cylinders. Anyway cut a long story short I changed spark plugs today to only find that the 5th cylinder was still brand new and piston still new. No carbon deposits. The issue turned out to be a loose injector adapter.

As this was a brand new engine and done about 500 miles on it, would the 5th cylinder be damaged at all when it wasn't running any fuel/firing?

Unlikely to be damaged. There were OEM engines that disabled cylinders in order to improve fuel ecomomy in low-load situations. It's just drag.

Now your rings on that cylinder may not be seated, a leak-down test would tell you for sure.

I will do a compression test in a few days and see. If my compression is low should I switch back to mineral oil (synthetic at the moment) and break it in? or is it too late for that with around 500 miles on the motor.