"conventional" doesn't really exist anymore, it is a synthetic blend now.So I actually called the shop up today that used to service her vehicle. They told me they always used conventional and she always did 3000 mile OCIs.
One thing I do notice about this engine that is interesting is the oil currently on the dipstick, even 1300ish miles later is very light. It looks like oil I just put in 300-400 miles later on most other cars. So I'm guessing there's very little sludge in the engine based on this. I don't want to pull the valve cover because they've actually been recently done and aren't leaking, I can see the RTV from whoever did them last.
I've been doing 5K conventional oil changes for years on most of my cars, should I keep this thing going at 3K like it was, or go to 5K? Oils used will probably be NAPA Syn-Blend, and Valvoline Maxlife, maybe Supertech again, but I figure go buy NAPA if it's on sale rather than support Wal-mart. NAPA Syn-Blend on sale is the same price as ST ($16 per 5qts) on sale.
3k oil change intervals is good so it shouldn't have a sludge issue even using synthetic blend.
My 1MZ-FE is doing perfectly fine with 5k intervals with full synthetic. I highly recommend against using a synthetic blend for 5k intervals on a 1MZ-FE.
Also do not forget to change the PCV valve like I mentioned before, it is only a few dollars.