Brand: Heavier oil can help high-mileage engines

Q: I own a 1994 GMC 3/4-ton van with a 350 engine. In the last year, the oil pressure has been low, just above the red, and the low warning indicator is on at idle. When I speed up, the pressure goes up. I change the oil every 4,000 miles. At the last oil change I changed it to 10W-40 although the manual calls for 5W-30; this seems to have helped the oil pressure. Is it OK to continue with 10W-40 oil? The van has 145,000 miles on it.

A: Yes. This is a practical method to improve oil pressure in an older, high-mileage engine. The slightly thicker oil film from the heavier base weight oil — 10W — can help protect worn engine bearings as well. As long as you're not hearing knocking or serious mechanical noises from the engine, the heavier oil should buy you many more miles of service from your vehicle.

Over the decades of buying and driving older, high-mileage vehicles, I've even used 15W-40 and 20W-50 oils in high-mileage engines to improve oil pressure and extend their service life.

Q: Our 2001 BMW 325xi has 154,000 mostly highway miles and runs fine except for using a quart of oil every 600 to 800 miles. This consumption started at something over the 100,000-mile mark. I've always used BMW oil or Mobil 1 fully synthetic oil, and change the oil and filter every 8,000 to 10,000 miles. The engine does not leak and I've never seen a whiff of blue smoke. I've replaced the oil separator valve with the "winter service" unit that apparently is now recommended by BMW. I also checked the external oil return line for sludge and I think it is clear down to the oil pan. Any suggestions other than new oil rings?

A: BMW's PCV system (positive crankcase ventilation) doesn't use a conventional PCV valve to regulate engine vacuum applied to the crankcase to extract and burn crankcase vapors and blowby. In your vehicle, BMW incorporated an oil separator into the PCV system to allow vapors but prevent liquid oil from being drawn into the induction system. Oil is separated out and allowed to drain back into the oil.

My first concern with your engine would be some type of obstruction or leakage in the PCV system that is preventing crankcase vapors from being drawn into the induction system. In this case, the crankcase could become pressurized with blowby and force oil past the oil control rings into the combustion chamber. Any type of air leak into the engine from the oil filler cap, dipstick, valve cover, etc., would tend to dilute PCV function and could increase oil consumption.

Similar to the first question in this column, you could certainly try a heavier synthetic to see if it helps oil consumption. As long as the engine runs well, oil is far less expensive than new rings.

Q: In 2007 I purchased a 1998 Buick Park Avenue with 111,000 actual miles; it now has 151,000 miles. Within days after the purchase I discovered that occasionally when accelerating from a stop, the transmission chatters like a stick shift driven by someone just learning to drive. When I let up on the gas, the chattering stops and shifting seems normal. It doesn't happen very often and doesn't seem to be related to how hard I hit the gas pedal. Do you have any idea what may be causing the chatter and do you think I should have it overhauled?

A: GM service bulletin #00-07-30-002B from 2002 identified possible causes for intermittent slippage/shuddering from the 4T65-E transmission in your vehicle. Sediment in the pressure control solenoid or valve body and incorrect oil level all can potentially cause fluctuations in hydraulic line pressure in the transmission.

Before you even think about an overhaul, try adding half a can of Sea Foam Trans Tune to the fluid, drive the car for several weeks, then consider have a complete flush, filter and fill.