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Engine Running Hot

2002 Subaru Outback 4cyl, 115000 miles in Central MA. After a recent oil change to synthetic, 5w30, the car seems to run much hotter than it used to on hot days (near red zone). Coolant levels are fine, oil level is fine, engine doesn’t seem to be burning oil. Could the switch to synthetic or the oil weight have something to do with this, or is it something else? Manual recommends 5w30 up to 100F, 10w30 or 10w40 for higher temps, but we haven’t seen 100F this summer (close though). Thanks.

No, the synthetic oil has nothing to do with this situation.

Unfortunately, the odometer mileage puts you in the ballpark for head gasket problems.
Check your coolant overflow container for evidence of oil seeping into the coolant (a heavy black line at the upper fluid level of the overflow container).

Also, pull the dipstick to see if there is evidence of coolant contamination of the motor oil (it will look like a chocolate milkshake).

Oil seepage into the coolant is not a fatal event, but it needs to be monitored.
Once you have coolant seeping into the motor oil, the engine’s days are numbered.

Also–check to make sure that the radiator cooling fans are running when the engine is hot. If not, that would be a much better scenario than a head gasket problem.

However, you need to get this attended to yesterday. Aluminum engines do not take well to overheating, and the next step will be warped cylinder heads and posssible other problems.

Well, no evidence of oil in the coolant, and no evidence of coolant in the oil. All fans are running. The engine running hot has happened only on the three hottest days of the summer (94F or higher). Could it be the switch from 10w-40 to 5w30 in the middle of the summer?

Get your cooling system looked at before you ruin or damage the motor. Running near or in the read zone on temp guage is bad news for a Subaru engine.

good luck.

Your car could care less about oil as long as it is filled to proper level and changed on a regular basis.

It’s not the motor oil and you really do not need to be running this engine to near the red zone. Subaru uses an all aluminum engine and they do not tolerate overheating very well.

Regarding the overheating, when does it do this?
A/C on. off, or both?
In town driving only, highway only, or both?