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My top up warning has come on after 9,000 miles (max 1.5L). The recommended brand is Castro Professional 5w30, but do I need to use that brand or will another recognised brand's 5w30 mix well enough? I'll try to get Castrol, but need to know in case I can only get another brand.

Any recognised brand will be fine as long as it's 5w30

And even the viscosity range will be irrelevant if you are just topping up. Any decent oil will do.

Thanks - Got some on next day from amazon 4L £38, but now feeling educated for next time

I think you need to be very careful about mixing different brands (or even same brand variants) of full synthetic oils as the additives can be different. What is important is to ensure you use the same spec (VW504 - VW507) as well as the same viscosity. For sure, a small quantity top-up is far safer than running at a low oil level but only for short period pending an oil change. Therefore try wherever possible to use the same top-up oil as currently used. Generally, if you wish to change brand or same brand variant, it should always be the correct spec and be done with a complete oil and filter change

Viscosity is not as important as the VW specification. 504/507 additive packs are very similar as are the crappy GIII base stocks still being used, hence there are no issues with additive compatibility. If using the 504/507 (which virtually every approved dealer is using in every engine now) then filling with the same spec oil from another brand won't be an issue.

wheres oilman when you need him....?