Differential Oils - can they be mixed?

I have an E46 that had the differential oil changed; unfortunately it was not filled up to the right capacity and now needs to be topped up by about
450ml of extra oil! I do not have the original oil with which it was initially used (Amsoil SVG 75w90) but have Castrol XO 75W90 (which is recommended by BMW) with which I am planning to top it up with asap. I have already done around 400 miles like an idiot and hope this hasn't caused much damage.
Are there any potential issues with combining diff. oil in particular, or would I be safe to go ahead?
Thanks.
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John wrote:

In general - oils are supposed to be compatible.
But - given the magic special properties of Amsoil - that "in general" may not be true. You may want to contact Amsoil on the subject.
BTW - where did you get Castrol XO? Are you in the US? BMW specified this for their motorcycle rear-drives and it is about impossible to obtain in the US..
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Generally but diff oils are usually different specs for normal and limited-slip due to the friction requirements of the clutches etc.
As an example FORD GB specced Castrol EP 80 for normal rear axle diff in the cars but if one fitted a limited slip "Salisbury" diff then the specc went to FORD EM2C22B whatever that was but I'll say that with normal oil the limited slip clunked and clattered and shook the car but with correct oil dead quiet and very effective.
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