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Oil weights and Florida hot weather

My friend says that since we live in Florida, and the cars deal with the extreme heat that even though the manual says to use 5-20 or 5-30 weight oil, for older cars, 100000 miles or more, you should use a heavier weight oil.
He suggests a 20-50 oil, saying the lighter oils thin out too fast and won’t stand up to the heat.

I am reluctant to buy into this and stray from what the manual says, but what he says makes sense. I was wondering if this sounded logical to anyone.

Use What The Car’s Manufacturer Recommends (Requires). It’s Often Written On The Engine’s Oil Fill Cap. Better Yet, Look In The Owner’s Manual.

Sometimes, there are temperature ranges for different viscosities. You could then use the “heavier” oil year-round in Florida.

Example: Use 5w30 oil at temeratures above 10F.
Example: Use 5w20 oil at temperatures below 10F and up to 100F.

Thinner oil helps your car at cold starts, but your car’s cooling system should keep the engine operating at similar operating temperatures, regardless of where it’s driven and what climate it’s in, once it’s warmed up.

I’d go with the engineers rather than good intentions.

I’ve had several times the mileage you’re talking about on many cars, just by using the recommended oil.


Stick with the manufacturer’s recommendation. If 20W50 is within the recommendation, use it; if not, don’t. I have been using the “thinner” oils for several years now, in similar heat conditions, without any negative repercussions. I don’t think the heat temperatures I face is that much different than yours in Florida.

If the engine isn’t burning or leaking oil then stick with what the manual recommends.
Modern oils hold up against high heat and stress even though they’re thinner than the oils of yesteryear.
That said, in Florida I would go with 5w-30. It MIGHT be a little better.
20w-50 will not flow as well at a cold start and may not protect the engine in the first few seconds.
If the cooling system is in good shape a modern engine’s oil will get no hotter when it’s 100F outside vs 50F outside.

My car, a 1998 Civic, has spent its whole life in Florida, and it has only used the Honda-recommended 5W-30 oil. I’ve had no oil related problems.

slobber, I would use 5W-20 if I had your car. Following your friend’s advice could ruin your car, and will definitely lead to lower fuel economy. Using 5W-20 will save you gas, and it will protect your engine.

Forget 20-50, that’s for Harley’s… but 10W-30 in Florida won’t hurt anything…Or even 15W-40 in a tired old engine…

These graphs might help make sense of it all.

The site has a great deal of information on oils.

And in an old, large domestic engine 15W-40 has been a very good oil in hot summer summer weather in the south. But the smaller engines with crankshaft driven gerotor style oil pumps 5W-20 will hold 75psi + at idle with the ambient temp in the upper 90s after hours of high speed driving. I was skeptical of the oils and checked it myself.

I’d use 5W-30, maybe 10W-30 in the summer. I’d NEVER use 20W-50 in a modern car, especially a Honda. The bearing clearances are sometimes too tight for 20W-50 to lubricate properly. Also, sometimes variable valve timing systems can be thrown off kilter by using oil that is too thick.

Even in hot weather, the car has a thermostat that controls the engine temp which is much higher than the ambient air temp; even in Florida heat. No need to deviate from oil recommendations as per the owner’s manual.

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Double Clutch Brings Up A Good Point. "Also, sometimes variable valve timing systems can be thrown off kilter by using oil that is too thick."
I Have Several Manufacturer’s Technical Service Bulletins Alerting Technicians To Engine Operating Problems Caused By The Wrong Oil Viscosity.


I think the best of all worlds would be a good 0W30 synthetic. It provides for easy starts, rapid oil circulation when warming up, and provides enough film strength for those hot days and exteme underhood temperatures.

The reason I mention 10W-30 in the summer is that Florida gets awfully hot, and while the thermostat and radiator fan will control the coolant temp, the oil can run a bit hotter than normal. In these conditions, I think that 10W-30 might be a bit better than 5W-30. Given that it could already be >90° when the car is started in the morning, cold-start temperatures aren’t really a concern. I’ve read that the greater the spread between the W viscosity and the hot viscosity, the more additives are needed to give the oil the proper viscosity index.

isn’t 20-50 like diesel engine oil?

Harley Davidson specified 20W-50 for years. With ambient temps remaining above 80*F they recommended 60W. My source is a late 70s factory shop manual.

5W-30 oil will behave as a 30 weight when the engine temperature rises.
That’s what a multi-grade oil does.

Rod Knox: “Harley Davidson specified 20W-50 for years. With ambient temps remaining above 80*F they recommended 60W. My source is a late 70s factory shop manual.”

…and Harley Davidson’s solution to the rear cylinder overheating was (and still is) to shut the rear cylinder off during low speed operation. Can we stop comparing an air cooled engine that is known for roasting riders’ chestnuts to a car with a liquid cooled engine?

“Can we stop comparing an air cooled engine that is known for roasting riders’ chestnuts to a car with a liquid cooled engine?”

Go ahead and stop. I thought the point about the 20w50 that was recommended for use in a car (see original post) being used in ambient dependent air cooled motorcycle engines was relavent. Particularly since cars have thermostat controlled liquid cooling systems.


Excuse me Whitey…

I have had to change my opinion of motor oil viscosity. When I first owned a car in the early 1960s, there were automotive people who advised against using detergent oil and multi-viscosity oils. Tom McCahill, who wrote for Mechanix Illustrated and published a book titled “What You Should Know About Cars” said “I prefer detergent in the bathtub, but not in my engines”. He called 10W - 30 motor oil “sucker juice” and said that 10W - 30 was “a lousy number 10 and a lousy number 30 weight oil”. My first car was a 1947 Pontiac. I used straight weight non-detergent oil. My next cars were a 1955 Pontiac followed by a 1954 Buick. I used detergent oils, but not multiviscosity. My next car was a 1965 Rambler. Its owner’s manual said to use 10W-30 so that is what I used. It recommended an oil and filter change every 4000 miles. I followed the manual and had no problems. My 1995 Ford Tempo called for 5W-30. My present ride, a 2011 Toyota Sienna calls for 0W-20 synthetic. I have driven it 19,000 miles in extremely hot and extremely cold weather. It uses no oil between changes. I talked to the service manager (not the service writer) at the Toyota dealer. He assured me that this oil is fine. He is an older man and understood that I grew up when we used 10W in the winter, 20 weight in the spring and fall and straight 30 in the summer. I may be an old geezer, but I’ll go along with the times.