Originally Posted By: sunruh
cci,
consider this:
you have a fill of M1 in your motor or tranny or diff.
dump it
then fill with Pennz Platinum.
tell me you got every single drop of M1 out before the PP went in.
what no you didnt? yes, you just mixed.
everybody mixes some on a change.
Quite right.
There is usually about a cup full of residual oil left on the insides of the engine/transmission assembly, even after an extended period of draining when changing your oil is hot.
On some engine designs, there can also be another 100 ml of oil trapped inside the oil scavenging pump, if a conscious effort isn't made to evacuate it when doing the oil change.
Also on other engine designs, < 80 ml of oil is trapped at the oil pickup zone unless the cover is removed to check/clean the filter screen at the time of the scheduled service.
To the OP.
As mentioned above.
All oils that are API certified are required to be miscible with other oils that are also API certified.
So there is always that level of compatibility between them, to facilitate a top off as required anywhere one may find themselves.
If one is concerned specifically about issues regarding additive pack compatibility.
In small ways, the mixing of two different oil formulations/brands or types may not be the gold standard.
But it is almost inconceivable that any "real world issues" will ever be encountered with regard to mixing motor oil. Neither have I ever heard of incidences of issues.
In summary, it's not a problem.
As an aside.
I have had the occasion to observe through Used Oil Analysis, where one particular additive in one oil formulation which was previously used, actually took 2 oil changes over a period of 12,000 kms to be completely washed out of the machine, and to finally end up at the base line levels of the new brand of oil.