i used amsoil (synthetic) at my last oil change... driven 8,000 miles on it (haven't had time to change oil)

since then, i've added 2 quarts of amsoil (synthetic), then 2 quarts of generic motor oil (non-synthetic), then 2 quarts of pennzoil (non-synthetic).

Is it bad to use non-synthetic to add oil when i have full-synthetic in there, and also mixing different brands of motorl oil?

Thanks,

 

It's not really bad to mix motor oils, although I don't agree with your strategy. It is far better to change your motor oil more often and preferable to stick with similar additive packages.

Mixing types of motor oils isn't the same as mixing Pepsi with Coke. The additive packages each manufacturer uses is different and although they are not incompatible, they aren't the same. For example, Pennzoil uses some Moly as an anti-wear additive, while Amsoil uses more ZDDP. Does that mean that a 50/50 mixture is as good as each one individually? This is not my field of expertise, but it wouldn't surprise me if the properties were not cumulative.

But the bigger concern is going 8000 miles on an engine that is apparently consuming oil and not getting it changed. The amount of blow-by that you are experiencing suggests that you are probably getting some oil contamination from combustion and you're not getting rid of it. You might be getting some fuel dilution, an increase in insolubles, and (depending on your driving habits) reduced TBN.

For the amount of money you've spent using Amsoil, I feel that your engine would be much happier using a conventional oil like Motorcraft 10W30 changed at 4000 miles than going 8000 miles on your blend. Also, knowing that your engine is thirsty for oil, I suggest that you buy a few extra quarts and keep them in your trunk.