Gear oil

[laney]

Hi all just a quick question can you mix synthetic with semisynthetic gear oil I ask as I have 3 liters of Penrite 75-90 and 2 liters of Penrite 75-90 semisynthetic both gl5 and mt1 thanks

[Captain_Rightfoot]

You can absolutely mix it. Will it be useful afterwards. That is the question.[bigsmile]

What are you using it in? You could call penrite on Monday.

My feeling is it would be fine, especially in something like a diff. #IMNOTAMECHANIC

[d2dave]

I don't reckon that it would matter mixing the two.

Lets say you used synthetic and you had a leaking flange seal.

You take it to a workshop and get leak fixed. They will then top up with what ever they keep in bulk, possibly non synthetic.

[Bohica]

Grok says it is fine, the only caveat is not to mix Polyglycol (PAG or POE)-based synthetics → These are not compatible with mineral oils (they can separate or form sludge). PAG gear oils are uncommon in regular automotive use — they're mostly in industrial compressors, some worm gears, or specific food-grade applications. If your synthetic says "PAG" or "polyglycol" on the bottle/SDS, do not mix.

Just check the label.