I have 4 quarts of Castrol and 1 of Penzoil. Same viscosity (5W-30, if I'm recalling correctly) and both are synthetic blends. Would there be any problem in using all 5 together on my next oil change? I do 5,000 mile changes and I'm due for one. I'd rather just use up the Penzoil that's been sitting on my shelf.

Along those same lines, I was planning to change the rear differential fluid too, but I realized last time I (incorrectly) used 75W-140 and still have a quart left. This time I brought the correct 75W-120. Would it to any major damage to mix a bit less than 1 quart of the old stuff with whatever amount it takes to fill up the pumpkin with the new stuff (I think it takes something like 5 or 6 quarts total)? I'm not entirely sure I got the viscocities right here. I brought the stuff a few weeks ago and my memory is a bit hazy, but the fact still stands there's 1 quart of thicker, incorrect, fluid and 5 or 6 of whatever the correct viscocity is. Both are Valvoline too.

Thanks for any advice.

Darrell

 

youre ok with mixing, but what's up with 75 120, you have a Chrysler 9.25 so that takes 140, if you had an earlier Dana 60 it would be 80 90 or if its an earlier 9.25 75-90

Edit just saw you said you weren't sure on the numbers.

Look into amsoil oils and lubes for future use.