What kind of engine oil

Dandy question, but your concern should be for going the other way and using old oil you have sitting in your garage in a newer engine. In the U.S. There are two ratings to look at on the container. One is something like "SG" and the other is "CD".

The "S" in that rating stands for "spark ignition", meaning a gas engine. The "C" in the other rating is for "compression ignition" meaning diesel engines. Engine manufacturers specify the minimum rating required for their engines, and that will be something that is currently available. Whenever a major improvement is made regarding the additives in the oil, the rating gets bumped up to the next second letter. That means an oil with a rating of "SG" has some improvement over an oil with a rating of "SF". Oil produced with the next significant upgrade will be rated "SH". You can use any oil with a rating higher than what was the best the year your engine was produced. If your engine calls for "SE", for example, you can use "SF", "SG", and whatever is higher than that. You probably should not use "SC" or "SD". If you still have some of that, it probably would not hurt to use one container at a time during an oil change to use up your stock. You will not find the older oils in an auto parts store. They always have oil with the current highest rating. An oil that is advertised as being for older engines or high-mileage engines will have more of some additives such as seal conditioners, but those will still work fine in engines that call for that rating.

The additives in question include things like anti-foaming agents, corrosion inhibitors, seal conditioners, dispersants, detergents, and viscosity index improvers. Many people seem to have a bigger problem when they switch brands of oil. Each brand has its own proprietary list of additives that work well together, but some may not be compatible with some additives in another brand of oil. For example, the seal conditioner in the old oil might be attacked by the detergent in the new oil. That could result in a leaking seal. When you drain and refill five quarts of oil during a normal oil change, there is actually about seven quarts of oil in the engine. Two quarts is stuck in the passages and bearings and that does not drain out. That means you still have two quarts worth of old additives after the oil change that may not be totally worn out and ineffective yet.

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Cheers, Ken