Ive been told Its bad to change from one oil to the other or visa /versa.I changed to castrol syntek blend in my 98 chevyS10 about 5 oil changes ago before I heard this advice.it got better mpg and power right away and the mpg is still there.leak at front seal got worse.last change I put in mobile syn high mileage/mob filter.I now have 142000 mi and perf. is getting a little worse.(I think i need a new timing chain) but may just get another truck soon.Im about to put conventional Penzoil High mileage oil and fram filter w some time release additive built into the filter for older engines.Is this a bad idea after the synthetic oil? Ive gotten my money and more out of this truck already and dont plan to spend alot on it anymore.I can change the chain myself.it might need new lifters etc. also though.any chevy experts out there?its a 4.3 v6 throttle body and I think towing my boat is taking a toll on it now.has this motor reached its limits.its the first chevy ive owned in 28 yrs.72 Blazer I wish I still had now!