I have a 760 Turbo 1988 with 107,000 mls. Have been using Castrol

GTX 10w 30 natural oil all along. Last oil change my mechanic

switched me to Castrol GTX 10 W 40 natural oil. Now I get a

blue exhaust smoke about 3-5 mins in idle after engine temp is

normal. Car was not overfill. Usually for about 800 mls I would tap

up with 1-pt of oil. need an oil change now and thought of going

back to the 10W 30 but with a syntetic blend and then next oil change

to a pure syntetic ( Mobile 1 )because of mileage and Turbo. Car

does not leak oil. Original owner. Recently replaced timing belt/

3 seals, heater coolant valve and coolant hoses. Turbo is original.

Any help on what's causing the blue smoke and what kind/grade oil

to use for NY winter and summer months?

I'm with John--it is most unlikely that the change from 10W-30 to 10W-40 would cause any problem in any way.

Sounds like the heavier oil blew out a weak turbo seal. It was probably already on it's way out, and this just helped things along.

A blown head gasket could also be the culprit. The main oil feed to the head runs up a galley between cylinders #1 and #2 on a B230F.

A broken ring would cause similar problems.

Pull the plugs and see if they are all sooty or if only one is. If they are all oil-fouled a turbo seal would be my guess; otherwise suspect a blown head gasket (if cyl. #1 or #2) or a bad ring in that cylinder.

The other possibility is the mechanic tromped on the gas and spun up the turbo before the new oil filter had a chance to fill up and get oil to all the parts, and smoked a few parts, but you'd really have to piss your mechanic off to do that.

As for the grade of oil, your owner's manual is the only place I'd trust. I've been satisfied with Pennzoil, but I found Castrol causes a varnish buildup. Personally, I've never bothered with synthetic. I've always believed the only way to get rid of dirty oil is by changing it often, not by using more expensive oil. I can't justify the cost of dumping 4.5 liters of synthetic every 5000km.

oil grade prob irrelevant to smoker. Sounds like turbo seals. I'd be surprised if going back to thinner oil helped the seal problem.

OTOH, it wouldn't hurt to run a drill bit through the pcv elbow...follow the vacuum line back from the manifold to the pcv elbow at the intersection of the 1/2" hose from the intake hose (biggish hose with foam insulator hose on it) across the cam cover.