I have two questions, when you start using full synthetic oil can you go back to conventional or non full synthetic? Also, some of my customers cars use lets say 5w20 and puts a totally different oil in like 20w50 or 10w30 etc saying it has high miles or other reasoning. I advise them to stick with the oil that the car requires, is this correct and what are your thoughts on this. I work for Advance Auto and the customers really amaze me how poorly they care for their cars. Wrong fluids, cheap parts etc. Lol
Stick with the oil viscosity recommended by the manufacturer unless it's a very old (like 1970s vintage) vehicle. Modern engines are built to very tight tolerances and have viscosity-sensitive components such as the variable valve timing mechanism.
You can go back and forth between conventional, synthetic blend, and synthetic as long as the manufacturer does not require synthetic oil. (It is always best to stick with synthetic in turbocharged applications due to the high temperatures encountered.)
I have two questions, when you start using full synthetic oil can you go back to conventional or non full synthetic?
Yes.
Also, some of my customers cars use lets say 5w20 and puts a totally different oil in like 20w50 or 10w30 etc saying it has high miles or other reasoning.
You're prematurely wearing out the engine if you do that. 20W-50 is way too heavy of an oil for a modern vehicle. Variable valve timing and such are critical for a modern engine, and using the wrong oil will mess things up. It was designed specifically for that viscosity. Only use the manufacturer's specifications and use a high mileage formulation after around 100k miles.
