Was in an auto parts store today and thy said it was ok to mix synthetic oil with regular oil?

I can't understand why so many people want to put absolute confidence it whatever "THEY" say, when "THEY" can't even be identified. Or if you feel safe following "Their" advice, go for it. Or better yet, If you're willing to trust the mechanical needs of your automobile to typed messages on the internet, more power to you. Personally I'd take my car to my mechanic and have my oil changed out for fear of all the unknowns that could possibly happen from blending oil and synthetics. Best of luck to you and your car with your decision.

Don't do it either use one or the other. If the bike has been ran on conventional oil it will not run as well on synthetic and vice versa, due to what the clutch has been broken in with. In a car engine you can usually switch oil types with no side effects, but not bikes! Also the additive packages in the oil are not the same and why try a chemistry experiment and possibly damage the clutch on your bike? Remember conventional oil has been refined from crude oil from the ground Synthetic is just that synthetic which equals man made 100%.

It's okay, but not really suggested. Because if you have synthetic oil in car, then adding regular oil with thicken it up and make it loose some of its special synthetic lubricating ability.

It's worse to mix oil weights (20W50, 10W30, etc). Engines nowadays are very specific. Their tolerances are marginal, meaning the engines are made so that there's little room for mistakes or changes. If an engine is made to use only 20W50, then that's what you should use. If you use a different oil, the motor will run, but not to the best as it was designed.

In old days, 50 years ago, cars weren't built so precisely. So you could mix oil weights with no problem. But modern cars are much more needing specific oil weights.

Now, if you've got regular oil in car and add some synthetic oil, as long as it's same weight, then that helps the regular oil a little bit, just a little bit, by thinning it so it moves easier in motor.

But it's just better to keep same kind of oil in engine and not mix oils or weights. YOu can mix brands, but even then, it changes the oil that was originally in the motor. I have found that Castrol regular oil works great for my old car, even gives me couple extra mpg. I used to use Walmart brand oil and it was okay, but Catrol works great in my car. Now, if I put a qt of the walmart oil in all my castrol oil, it'd not get me as good of performance if I had only Castrol oil in it.

It's just better for the machine if you kept the oil the same. Whether different kind, weight, or brand.

But yes, you can add synthetic with regular oil.

Yes, they are fine to mix. There is no big advantage to mixing but no harm either.

All modern motor oils are cross compatible. You can safely mix synthetic with conventional, mix brands, and mix grades (5W-30 with a 10W-40, for example). There is no logical reason to do that but if you are just topping off your oil and need to use whatever, that's fine. Stick with the good brand name oils and you won't have a problem.

Here is a total news flash. Today, all IMPORTANT oils are car specific. If you READ the labels, on the back it will tell you which cars the oil was designed for. For example, oil for VW requires oil that meets specification 502/505. Oil for Asian cars must meet JASO specs. If the oil is for GM, it will say so ON THE BACK of the container. SEE??? Chrysler and Honda, in general, use the same oil "qualities". Get it??? by the way, it IS ok to mix regular with synthetic. My advice, though, is to stick with 100% synthetic since it will NOT make sludge or cause lubrication issues later on. Regular oils ALWAYS do! GET SMART, and read the LABELS!!!!

I would say that 1qt regular oil to 4qts of syntec would be ok but if I was doing an oil change I would try to stay with

Only one kind of oil.

I only put synthetic oil in my truck. I was told keep it synthetic or regular. Don't mix.

You can buy synthetic blend so why would it not be ok the mix.......md