Topped up Engine with Gear Oil! - Problem?

Dec 14, 2021
I've got an 06 S2000. After thinking I was done with autocrossing for the year, I signed up for one last event. I had already put my stuff away and had to throw together my box of crap in the morning. Unfortunately, I grabbed the Gear Oil instead of the Engine Oil. Darn Ams-Oil putting both in the same bottle. I run Ams-Oil signature series 10W-30 normally. Gear Oil was Ams-Oil Sever Gear 75W-140.

Right before I went to run, I checked my oil. Oil was a little on the low end of the acceptable window so I went to top up just to be safe. I started pouring and realized very quickly the fluid didn't look right. I only put in less than 1/10 a quart before pulling it off. My oil capacity is about 5 quarts. Therefore, the mix had about 2% gear oil.

I was a few hundred miles from home. I ran the event as normal and then drove home for a total of 500ish miles. I changed the oil 6 days later and dumped an extra quart through the pan to clear things out. This change had about 6200 miles on it total. I ran the interval longer based on a previous BlackStone UOA.

Any concern here? I know there's some concern about cold starts with thicker fluid, foaming, different additive packages, and possible acidic build-up eating yellow metals. Given that I only put 1/10 a quart in and ran it for 6 days, is there really any reason to worry? I checked at the dipstick and oil fill to make sure no obvious foaming was happening. I also did see that synthetic gear oils will typically not become corrosive as quickly. I think I only had 4 cold starts on it, with the gear oil in it too. I was a bit paranoid but I think it's probably of no concern. I am sending out a sample to Blackstone as well.
No big deal honestly, just not optimal. The pressure additive that smells wouldn't cause any harm to the rest of the oil. its still oil made from similar base stocks with some additives and antioxidants.
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And as an autocrosser you should be more careful. Why not ask Amsoil since that's what you used?
Won't hurt a thing , why would you think it would cause a problem? I had a work van and the newer vans has a sliding rear door and I preffered the doors that swing open . The engine had a rod knock . The company owner would have pulled the van from service and gave me a new van . For 3 years I added gear oil to thicken up the oil and reduce the rod knock. Granted this was in the SF Bay Area where then thicker oil wouldn't cause problems with cold starting pumpability.
We had a recent discussion on this topic:

and in terms of lubrication chemistry, it is a terribly bad idea.

I would do a drain and refill ASAP after the race.

Extreme Pressure additives found in gear lubes have a detrimental effect on engine oil.
Not sure I'd do that in a car but my lawn mowers and snow blowers run on drip oil, and that includes lots of gear oil.
Curious that in 2021 people on this board consider gear oil "thicker" than motor oil.

OP, it would have been better to run the few oz low than top up with gear oil but, what's done is done and it's not like you used a whole quart.
TY for the replies. I did see the thread about the diesel engine. I would not do the same thing but he did make it to 800k miles.

It has been changed and flushed with an extra quart. As to being careful, I am. I've been racing for years and never done anything dumb like this. Ams-Oil changed their severe gear bottles to black with blue caps to match the oil bottle I use. In every other change over the course of years, Severe Gear has had a black cap and the oil a blue cap. Easy visual difference that created muscle memory for me. I had 6 bottles (5 oil + 1 diff) waiting in my stash for the end of year changes and happened to grab the wrong one on my way out the door. I'm also paranoid enough to still be posting about it while I await my UOA. Trust me, I feel plenty dumb.

Asking Ams-Oil is a good idea. I'll have to see if they have a customer service line or something.