I sent an email to Castrol asking about mixing grades within the same brand and also mixing across their brands.
In short, the only thing they say is ok in terms of mixing, is topping off with the same grade of oil. If you do this a lot, make sure it's the same formulation. Anything else in terms of topping off and mixing is not recommended. Here are the key points of the reply I got:
- For topping off: You can use any of their products / formulations so long as it is the same grade. They do not recommend topping off with another grade.
- They do not recommend topping off with a different formulation on a continuing basis.
- Do not recommend mixing different grades even in the same formulation.
- Do not recommend mixing formulations due to changes in viscosity that can develop. (This is something Shannow pointed out).
- They said that Edge gold bottle with Titanium FST is not the same as the Edge gold bottle extended performance.
In short, the only thing they say is ok in terms of mixing, is topping off with the same grade of oil. If you do this a lot, make sure it's the same formulation. Anything else in terms of topping off and mixing is not recommended. Here are the key points of the reply I got:
- For topping off: You can use any of their products / formulations so long as it is the same grade. They do not recommend topping off with another grade.
- They do not recommend topping off with a different formulation on a continuing basis.
- Do not recommend mixing different grades even in the same formulation.
- Do not recommend mixing formulations due to changes in viscosity that can develop. (This is something Shannow pointed out).
- They said that Edge gold bottle with Titanium FST is not the same as the Edge gold bottle extended performance.