Maddman, you are an idiot.
DO NOT use the synthetic in the car unless you are planning on using synthetic from now on. You can't go back to regular oil after that. Just to let you know I build the Corolla in Canada and we use a semi-synthetic in the motor already. You need to use the 5W-20 or the 0W-20. It might void your Powertrain warranty if you change the oil. Not many people know that but if you put the wrong oil in the car that not is recommended if could damage your engine.
1. switching between types of oil will not cause engine damage execpt in the rare instance that one additive package from one brand does not react well to another add pack from another brand, running M1 full syth and running M1 extended drain conventional will not cause problems.
2. yes, you should use whats indicated in the manaul, however changing your own oil will not void the warranty. 10points of weight difference (20-30) will not kill the engine. if he was going to put in a change of 20W-50 then yes that would cause harm.
Hey Bitter, you gotta switch to decaf.
I will say that you are correct. Switching from dino to synthetic oils and back will not harm an engine. What do you think a semi-synthetic is made of anyway.all that discussion was of older engines that may have seals loosen and leaks occurring from switching to synthetic oils ,not engine wear.
Changing the viscosity of an oil will not hurt either unless you go extreme and put way too heavy oil in. remember that most of today's oils have a wonderful additive package. As long as your oil does not get old and crushed by wear , the viscosity will remain constant. If anything it will lower the viscosity over time.A 5-10 point difference in viscosity will do no harm.
Yes , the gas mileage will be slightly lower. I have proven that myself by using 10-40 high mileage to fill in when I burn a quart. My mileage goes down a point or 2.
No warranty will ever be voided by improper oil .The only warranty voids I see are for letting the car run on low or no oil. Who the heck is going to find out unless you put gear oil in and some dealership mechanic sees it.