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Oil grade

I have a 1999 Chevy Venture van with about 200,000 miles on it.I recently had my heads resurfaced and new gaskets installled. My mechanic suggested I use 40 instead of 10/30 weight oil since I have so many miles on the motor. Down here in South Texas we have pretty hot summers,100+. Winters are mild. It rarely gets into the 30, mostly 40s, 50s and lot of the winter is in the 60s and 70s occasional 80s. Is it ok to use 40 weight oil. I hope so because I told him to go ahead and use 40W. What do you think?

40 Weight seems a litle heavy for a modern engine. Given your climate, the age of your Venture and the miles on the engine, 40W probably isn’t going to hurt anything. When you first start up the engine, give the engine 30 seconds or so for the oil pressure to stabilize and circulate through the bearings.
Your 1999 Chevy Venture is like me. We’ve both got some age on us and seen a lot of service. At my age, I need a good dark Lager beer and I stay away from the light stuff. Maybe this is why your mechanic suggests 40W oil.

Just to clarify your mechanic did NOT put 40w in the engine. He put in 10w-40. There is no 40 winter weight (that I know of).

Did you check your owners manual?? It should be spelled out in the owners manual. It might give you a range of oils you can use depending on the ambient temperature. I suspect 10w-40 is fine.

With that said…it only looks like you had a head gasket replaced. This has NOTHING to do with the weight of oil. The 10w-30 oil got you to 200k miles…I don’t see any reason it can’t get you to the next 200k miles. My pathfinder has 340k miles on it and I still use 10w-30…Doesn’t burn one drop of oil either.

I think I have seen 40W oil in recent years at a farm store chain called “Rural King”. It may be intended more for farm tractors than cars. Years ago, we had 40W oil around that we used in oil bath type air cleaners. The specifications for the air cleaners called for 50W oil, but that was hard to find, so we took the next best thing. It probably didn’t matter what viscosity oil we put in these air cleaners, but we tried to follow the label.

Sure it wasn’t 40 - Weight???

The W by itself indicates Winter. 40w would be awfully thick when temps got below freezing. I’d be surprised if it would even flow.

I would say that for a van that old, 10W40 would be just fine, but NEVER straight 40. That is used by the marine and railway industry for Heavy Duty large diesels. Locomotives are seldom shut down and marine oils are preheated before starting the engine.

I lived in a tropical country for 5 years and 20W50 was the most sold car engine oil. The choice of 10W40 is a good compromise. Just don’t vist your aunt in Minnesota in January.

You are right. It was 40 weight. Speaking of oil viscosity, I was riding back to campus with a classmate who had a 1950 Pontiac straight 8. A connecting rod started knocking. We were 20 miles from campus and stopped at an all night service station. The mechanic drained the oil and put in 90 weight gear lube. We made the 20 miles back to campus. The next day a wrecker from the salvage yard hauled the Pontiac away.

My problem with 10W-40 oil, which may have been solved by now, is that it caused caron build-up in the 260 V-8 engine on my 1978 Oldsmobile. I was using 10W-40 as one of the oil recommendations in the owner’s manual. I was experiencing detonation problems (pinging) on acceleration. I was having to pour Casite Motor Tune-up through the carburetor every 2000 miles or so and was also adding the Casite to the gasoline about every third fill-up. A caller to Cartalk was experiencing the same problem and Click and Clack recommended switching to 10W-30 oil. Apparently the polymers in certain brands of 10W-40 in certain engines would lead to carbon deposits. I switched to 10W-30 and the problem went away. I have no idea what brand caused the problem as I would buy whatever was on sale. Maybe modern 10W-40 oil in modern fuel injected engines don’t have this problem.

The problem with 10w-40 is the additives. 10w-40 is really just 10 weight oil with polymers to make it behave like 40 weight when it gets hot. The wider range the oil has to work the more additives (i.e. NOT OIL) has to be added. It got to the point that many engine manufacturers specifically said NOT to use 10w-40. 10w-30 was the new preferred engine oil. The rule of thum is to keep the range to 20 or less.

BTW…this works differently for Synthetic oil.

I would not use straight 40 weight. I might use 10w-40 if I was trying to solve an oil use problem, but before that I would try one of those ‘high-mile’ oils for older engines in the weight specified by the manual. What problem are you trying to solve?

Would this argument apply to 5W-30 oil because the range is more than 20. The problem I had with carbon build-up using 10W-40 occured with a carbureted engine, so perhaps, since fuel injected engines are less apt to have carbon build-up, a wider visocsity range is not a problem.