So you saved your used gear oil in some container other than a large(er) waste oil container, and that container was so similar in shape and size that you confused it with a new motor oil container? Or, you keep your new motor oil in a "non-original" container that is similar or identical to the used gear oil container?Couldn't believe my self this morning. My van was low on oil and I've top it with USED gear oil. After two litre I noticed the gear oil smell . (Nonoriginal jug).Dropped the plug and made a OC without filter. Bloody stress!
That sounds like the automotive equivalent of a surgeon closing a patient leaving in a couple of sponges.