I have a 2001 Ford Explorer Sport 335,000 miles, automatic transmission, does it do any harm if I change from Castrol synthetic Blend 10w-40 to Mobil 1 5w-30 fully synthetic oil? It originally was used with 5w-30 but since 190,000 we started using 10w-40 and I wanted to use 5w-30 since it’s cold here in Texas
Use what the owners manual says in the engine. You've probably caused unneccessary internal wear by driving with that heavier oil in the motor for over 145,000 miles. That 2001 Explorer doesn't have a pre-1970s and '80s designed engine. Oil viscosity recommendsations were flexible based on temperature back then, not now. Metallurgy and oil are very different these days. Modern engines were made for one specific oil weight, nothing else. Put the right weight oil in keep an eye on the level. Changing to higher viscosities as an engine ages is an old truth, not necessary today.
Changing to full synthetic oil in a 21 year old vehicle is a mistake. Synthetic blend is fine. The oil weight should be based on the ambient temperature the vehicle is operated in. Consult your owner's manual for the correct information. I have an older vehicle and use synthetic blend. I use 5w30 in the cold, winter months and 10w40 in the hot summer months. The vehicle is now 41 years old. This is only my personal experience and open for debate.
